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Discussion Starter · #1 · (Edited)
Sp101 forcing cone (now more ?s), need technical advice

Hello, pretty new here and fairly new to Ruger revolvers but not firearms in general. The revolver bug has bit me pretty hard. I have been researching the option of cutting the forcing cone on my SP101 to an 11 degree angle because I have been experiencing fairly severe leading from my handloads. It seems this procedure is helpful in reducing this condition and may possibly help with accuracy. I know there are reloading options to lessen the problem as well and I am exploring those variables.

What has not been conclusive in my reading is how exactly this 11 degree measurement is taken. I can assume that the 5 degree measurement (of factory cone) is between center of bore and cone surface because I can take measurements and confirm by drafting it out. What I don't have is an 11 degree cutter in my hand to look at. So the question is: is the 11 degree a measurement from bore centerline to cone, or an "included" angle that measures from cone surface to cone surface. I am assuming that an 11 degree included angle cut is not what is correct because that would be so close to the factory cut that I can't see how it would help much. Please advise.

I am under the impression that opening up the largest diameter of the cone beyond what the Brownell's literature designates (.02 over caliber) is not a big issue. True?

The critical measurements in the drawing are fairly to scale and as precise as I could reasonably get for the purposes of my question.

I would also like to hear about your success or lack thereof after performing this operation. Perhaps I would be better off just honing the surface of my factory-cut cone? Pictures of your re-cut cone would be awesome too.

Thanks
 

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Clayton, Forcing cone angles are measured by the total angle. In other words, a 5 deg angle would be 2.5 deg on either side of center whereas as an 11 degree cone would be 5.5 deg on either side of center.

The primary function of a forcing cone is to compensate for slight cylinder-to-bore alignment issues. With jacketed bullets where the bullet diameter is the same as the bore diameter, a 5 deg cone is optimum. With lead bullets, the cone serves a second function and that is to size the slightly oversized bullet down to bore diameter. Because lead bullets are typically .001" larger than jacketed bullets, an 11 deg forcing cone is optimum and helps to size the bullet without distorting it. "Optimum" means minimal bullet distortion and minimum pressure loss. In most cases, an 11 deg cone will improve lead bullet accuracy and reduce bore fouling but only if a couple other conditions are met. You must use lead bullets that measure .358" and their hardness must be matched to chamber pressure. The cylinder throats must be .358~.3585".

If you chamfer your forcing cone to 11 deg and shoot jacketed bullets, chances are accuracy will not be affected, however you will lose more pressure from the B/C gap, which in turn will decrease muzzle velocity .... typically 50 fps compared to a 5 deg cone. SP-101s were designed primarily for self defense with jacketed bullets. They are not "target grade" but are accurate enough for their intended purpose. As such, most people do not alter their SP-101 forcing cones unless they exclusively shoot lead bullets.

Here's a document I put in the Forum Library that helps explain more about lead bullets in a revolver. Click on this link to download: http://rugerforum.net/library/19869-lead-bullets-revolvers.html
 

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You might inspect your cone for rough machining marks that can & do create lead buildup.
I've had them on a couple Rugers.
If they're present, they need to be cleaned up.
Denis
 

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If I were you, I would look at other issues involved with cast bullet shooting before I "remanufactured" the forcing cone. I have Brownell's complete set of tools and have serviced more than a few, but other issues were usually the at the root of leading.

First, make certain that your cylinder throats are the same size as, or possibly 0.001" over groove diameter. The throats should be reasonably smooth, should range correctly, and be uniform in size.

Second, the bullet diameter should be the same size as, or a little over the cylinder throat diameter. When shooting lead bullets, a little oversize is of no matter. Colt regularly made their Pythons that shot bullets from 0.357" to 0.362", both jacketed and lead, shoot through groove diameters of 0.355".

Your lead bullet hardness must match the pressure/velocity level of your load. More velocity means you need more hardness. The lube must be correct for the application, too.

Modern sizing dies are made to put a death grip on jacketed slugs. That, one supposes, so that the slow burning powders can "get it up" and burn well. This condition can be detrimental to lead bullets, where the tight case will resize a soft lead bullet down--in spite of your best efforts to get it right in the first place. Find an old sizing die as a second and polish it out to a larger diameter and leave the lead bullets their original size.

A decent crimp can, at times, aid cast bullet accuracy. The most useful seems to be Redding's Pro-Crimp and the least useful is a sloppy roll crimp. Taper crimp dies are a mixed bag for lead bullets--they sound good, but often perform poorly.
 

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pgeobc, I agree with everything you said with one minor exception.
Second, the bullet diameter should be the same size as, or a little over the cylinder throat diameter.
When bullets are larger than throats, the throats size them down anyway. The biggest problem with using bullets larger than throat diameter is a dramatic increase in chamber pressure. This has two bad side effects, one being obvious ... a possible over pressure condition. The second being excessive forward cylinder thrust where the cylinder tries to move in the same direction as the bullet. This will increase endshake wear in any revolver but especially in DA revolvers. Not good!

The worst crimp die you can use with lead bullets is a Lee FCD. That's because the insert in the die does a full length resize of the entire case. If you load a .358" bullet in a 38 Special or 357 Mag case, crimp it with a Lee FCD, then pull the bullet, you will find the diameter has been squished down considerably smaller than .358". Taper crimp dies (all brands) are intended for semi-auto cases but can be adapted to lead bullets in a revolver cartridge when the bullet does not have a crimp groove. This is common with 45 Colt cartridges when loaded with 45 ACP bullets (no crimp groove). With that token exception, using taper crimp dies with lead bullets is not a good idea.

DPris, Yes, I've seen my share of Rugers with rough forcing cones ... usually machine marks from a powered reamer that almost look like screw threads. A 5 deg forcing cone chamfer tool will clean up the factory cone with just a few light twists of the "T" handle. Instructions for chamfering forcing cones and cylinder throats can be found in the forum Library. Click on this link: http://rugerforum.net/library/18570-forcing-cone-cylinder-throat-chamfering.html
 

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You might inspect your cone for rough machining marks that can & do create lead buildup.
I've had them on a couple Rugers.
If they're present, they need to be cleaned up.
Denis
I would also suggest the above as I've shot Ruger handguns all of my adult life from my first 21st birthday handgun (a Redhawk and Security Six) to my 52 year old present collection of Ruger handguns. I've never had a problem with excessive leading with any of them including the black powder 45LC loads I was shooting in SASS. I did experience some front sight plating when I had a couple of them Mag-Na-Ported and used lead rounds. Lead would shear off and deposit on the sides of the front sight blade. Other than that, no leading problems at all. Smithy.
 

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Discussion Starter · #7 · (Edited)
First of all, thanks to all who have replied and contributed their knowledge and experience. There are a few facts that I did not share in my initial post that are relevant to the discussion that I'll share.

This issue actually applies to two SPs that I have- one a three inch and one four. I'm more interested in squeezing the most accuracy out of the four incher, since it lends itself to longer shots by nature of barrel length and adjustable sights. I do want to solve the leading problem in both of them, and optimize them for lead bullets. I am acquiring the necessary equipment to start casting bullets.

On both guns, cylinder bores allow a .357+ pin guage to just pass, but not a .358-, using what I consider to be a fairly reputable guages from McMaster Carr. All bores seem uniform to me. I may open them up as recommended.

Forcing cones on both are a little rough, with machine marks concentric to the bore that even extend onto the lands. Yuck. I will tend to this before considering a new cut. . Can a brass polishing cone from Brownell's handle this task? I should add that the leading is not happening on the cone itself, but rather in the grooves immediately past the cone

I am going to bump up the powder in my handloads. I've been shooting 158swc in .38 cases with minimum load of Bullseye.(Wimpy, I know, but my six year old son and I shoot together and he can keep these on an 8.5"x11" piece of paper at 7 yards-I'm very proud). Anyway, I hope a little more pressure may obturate the slug a bit more and provide a better seal. I don't have a lead tester, so I don't know BHN.

And, lastly: I have not yet invested in a bullet sizer/lubricator. I read some claims that they are not necessary. Are they a worthwile investment? If my bullet molds produce slugs at .357 and .358 would be ideal, is a sizer capable of increasing diameter?

Thanks
 

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Brownells does offer brass laps just for that purpose. One of those with a little polishing compound would do wonders. Shooting single action in SASS we were required to use only lead. No gas checks or jackets at all. I found the my bullets were consistently of different size and if I added an ingot in the middle of my cast the lead temperature would be changed and I'd end up with different sized bullets as a result. I went to getting a full pot up to temp and fluxed. Then with a preheated mold I'd cast the entire pot and never refill during the cast. With that I ended up with more consistent bullets but still varying in sizes. I finally realized that although cut with the same cherry, a six cavity mold will be slightly off between the first cut (or cavity) and the last. Sizing is a perfect way to even out the differences. Lee offers sizing dies in every diameter you could imagine and you can custom order an odd size from Lee for not much more than their standard sizes. A lot of folks have great success with a Lee sizer and using their liquid alox for lube. If you want a hard lube then pan lubing (a real PITB) or a lubrisizer is the only way to go. I ended up Lee sizing my bullets to one thousandth over final diameter and then into the lubrisizer for the last one thousandth and for lubing. Lot of work, but I had consistent results in the cowboy shooting arena. Smithy.
 

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Well, that was handy.

I guess you can work from the hardness supplied/implied from the supplier, or take the longer route of working up loads that do not cause problems. ?? I've used the slow way for years.

One question I ponder though. How are the properties of bullet hardness and propellant burn rate interacting?

I see different effects when switching from faster to slower propellants. It seems burn rate affects leading, independent of actual velocity. I'm sure obturation is the driver, but is there a solid "rule" to go by?
 

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TMan51,
How are the properties of bullet hardness and propellant burn rate interacting?
Let's start with the bullet. As you probably know ... lead is a very malleable metal, meaning it will change shape easily when pressure is applied. As an experiment, place a bullet base down on an anvil and strike the nose of the bullet with a hammer. This will deform the bullet and make it shorter. It will also make the bullet's diameter fatter. The harder you strike the bullet, the fatter it will get. That explains malleability. Now let's add "hardness" to the experiment. Pure lead is very soft .... about BHN 5. As alloy metals (antimony) are added, lead gets harder ... up to about BHN 26. If you conduct the same hammer and anvil tests with a BHN 5 bullet and a BHN 26 bullet, you will discover the softer bullet will expand in diameter very easily whereas the harder bullet requires a much harder hammer hit to expand the same amount. So ... the rule is: the harder the bullet, the more pressure you need to make the diameter expand. Keep in mind ... a modest 38 Special load will generate more than 5 tons of chamber pressure ... way more pressure than you could possibly apply with a hammer.

Now for the powder. I'll use Bullseye (very fast burn rate) and Unique (medium burn rate) powders for this example. Page 883 in the Speer #14 manual lists 158 gr lead bullet loads for a 38 Special. It only takes 3.5 gr of Bullseye to develop a muzzle velocity of 814 fps. Unique powder takes 4.7 gr to develop almost an identical velocity of 815 fps. In all cases, less powder for the same velocity means the powder burns faster. Using an equal 4.0 gr powder charge with both powders and the same bullet, we find the Bullseye load develops 12,509 psi chamber pressure whereas the Unique load develops 9746 psi (using QuickLOAD as a reference). This tells us a faster burning powder will always develop more chamber pressure than a slower burning powder with the same powder charge weight and the same bullet.

Faster burning powders develop high chamber pressures when they ignite but because the powder burns so fast, it is all burned up in just an inch of bullet travel. This condition is much like hitting a golf ball with a club ... very high initial energy but driving energy stops as soon as the golf ball bounces off the club head. Slower burning powders are more like a bow and arrow .... where string tension continues to drive the arrow faster until the bow string is straight. A slower burning powder takes longer to burn and keeps pushing the bullet faster and faster until the bullet finally leaves the muzzle.

Obturation: The process where chamber pressure causes a malleable bullet to shorten in length and increase in diameter until it reaches it's limit (bore diameter). The ideal situation is when there is enough chamber pressure to force obturation but not so much where pressure will vent and bypass the bullet. When chamber pressure best matches bullet hardness, the bullet will develop a good seal in the bore and will not allow hot expanding gasses to escape between the bullet's circumference and the bore. If a bullet is too hard or there isn't enough chamber pressure, it will not get a good bore seal. Likewise, if the bullet is too soft or if chamber pressure is too high, expanding gasses will vent between the bullet and bore. In either case, when pressure escapes between the bullet and bore, the very hot high pressure gasses will cause the circumference of the bullet to erode (melt) and turn to lead vapor. Most lead vapor exits the muzzle but some is left behind in the bore where it solidifies and becomes lead fouling.

So ... we can take advantage of powder burn rates by selecting the powder and charge weight that will generate the proper chamber pressure to force obturation. To do this, we need a vital piece of information ... bullet hardness (measured in Brinell Hardness Number or BHN). Speer bullets in the above reference are BHN 10. Using the formula: Chamber pressure=BHN times 1400, we find the optimum chamber pressure for this bullet is 14,000 psi. The max SAAMI pressure for a 38 Special is 17,000 psi so the optimum pressure is very safe and is well under max.

The next step requires some research. A software program called "QuickLOAD" can be used to determine the powder type and charge weight that will develop the desired chamber pressure. QuickLOAD has just about every powder you can find plus it has a huge data base of bullets by brands, types, diameters, and weights. I used QuickLOAD to find the optimum charge weight for Unique with a 158gr Speer .358" SWC bullet and here's the results: With Unique powder, the optimum pressure occurs with 4.8gr, which is almost identical to the Speer manual max load of 4.7gr.

So to directly answer your question .... the interaction of the bullet's hardness and the driving chamber pressure will take advantage lead malleability and cause the bullet to expand and form a tight seal in the bore. This minimizes bore fouling and maximizes accuracy.

It seems burn rate affects leading, independent of actual velocity. I'm sure obturation is the driver, but is there a solid "rule" to go by?
Actually, velocity has very little to do with bore leading as long as you match bullet hardness to chamber pressure (within reasonable limits). You can successfully get excellent accuracy with minimal fouling from 700~1500 fps without a gas check. If bullet hardness is mismatched to chamber pressure, you can get excessive fouling at any velocity but more fouling will occur at higher velocities just because more powder was used.
 

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Iowegan,

You wrote "When bullets are larger than throats, the throats size them down anyway. The biggest problem with using bullets larger than throat diameter is a dramatic increase in chamber pressure. This has two bad side effects, one being obvious ... a possible over pressure condition. The second being excessive forward cylinder thrust where the cylinder tries to move in the same direction as the bullet. This will increase endshake wear in any revolver but especially in DA revolvers. Not good!"

Although the throats do size them down, large bullets prevent gas cutting in the throat and, thereby, prevent leading from starting in the throats. It is quite easy to demonstrate that once leading starts, say in the throats, that it will continue doen most of the rest of the firearm. Although many have warned of the problem of high pressures, as you write, the problem is non-existent with lead bullets. After all, one can slug the bore with a lead bullet simply by pounding it in with a mallet. The real force needed to swage down an oversized lead bullet is on the order of only 500 lbs at the most. In fact, people resize them in sizer-lubricators with minimum effort all day long. That does not even attempt to cover or explain what happens with even the hardest lead bullets in, say a .454 Casull, or other very high pressure gun, where the bullets are known to ecpand up going between the cylinder and the barrel (mostly limited by the taper of the barrel forcing cone) and then swaged back down to fit the groove.

Even copper jacketed bullets can be shot modestly oversized without the world coming to an end. After all, many service rifles, from countries with poor quality control, that have widely varying bore and groove diameters, are shot with standard one-size-fits all ammo. An good example of this is the SMLE in .303 British. Ammo is very much standardized at about 0.311" for bullet diameter, but barrels vary from 0.309" to 0.314" and more, especially during wartime production.

"The worst crimp die you can use with lead bullets is a Lee FCD. That's because the insert in the die does a full length resize of the entire case. If you load a .358" bullet in a 38 Special or 357 Mag case, crimp it with a Lee FCD, then pull the bullet, you will find the diameter has been squished down considerably smaller than .358". "

Well, sort of. That is why I had Lee make me custom FCD dies in different diameters. They will do that for a very reasonable price. Still, the standard Lee FCD is not geared toward producing minimum ammo, but at simply making certain that a maximum catridge will be the limit. I really do not like sizing down bullets with either tight cases or with post-seating devices, but I have developed a method that uses the Redding Pro Crimp Die to do post-seating crimp and size/taper .38 Special HBWC ammo. In every revolver in which I have tried the method, it has improved the precision (group size) of the load considerably. Often, the improvement is on the order of a 40 percent reduction.
 

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Just a side note: Got my SP101 back from Mag-Na-Port today and did a poor man's action job on it, deburring interior parts, polishing action parts and replacing springs with a Wolff spring kit. Assembled the gun including its new Nylon Hogue grips. With its Safariland inside the pocket holster the gun disappears inside my front pocket. I thoroughly checked out the gun and can say that there are no machining marks or dings anywhere on the gun including the forcing cone. Smithy.
 

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pgeobc,
Although the throats do size them down, large bullets prevent gas cutting in the throat and, thereby, prevent leading from starting in the throats. It is quite easy to demonstrate that once leading starts, say in the throats, that it will continue doen most of the rest of the firearm. Although many have warned of the problem of high pressures, as you write, the problem is non-existent with lead bullets. After all, one can slug the bore with a lead bullet simply by pounding it in with a mallet. The real force needed to swage down an oversized lead bullet is on the order of only 500 lbs at the most. In fact, people resize them in sizer-lubricators with minimum effort all day long. That does not even attempt to cover or explain what happens with even the hardest lead bullets in, say a .454 Casull, or other very high pressure gun, where the bullets are known to ecpand up going between the cylinder and the barrel (mostly limited by the taper of the barrel forcing cone) and then swaged back down to fit the groove.
These comments couldn't be farther from the truth. A bullet just .001" larger than throat diameter can increase chamber pressure by several thousand psi. It has nothing to do with how much force is required to size a bullet in a resizing die or to push a slug down a bore. You can push snow out of a muzzle with a soda straw but if the gun is fired, the barrel will peel back like a banana. Further, your comparison to a SMLE in .303 is about as far away as you can get from the function of a revolver .... no forcing cone, no cylinder, no B/C gap, no lead bullets, etc. You might want to do some research on the subject before posting.
 

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pgeobc,
These comments couldn't be farther from the truth. A bullet just .001" larger than throat diameter can increase chamber pressure by several thousand psi. It has nothing to do with how much force is required to size a bullet in a resizing die or to push a slug down a bore. You can push snow out of a muzzle with a soda straw but if the gun is fired, the barrel will peel back like a banana. Further, your comparison to a SMLE in .303 is about as far away as you can get from the function of a revolver .... no forcing cone, no cylinder, no B/C gap, no lead bullets, etc. You might want to do some research on the subject before posting.
Iowegan,

Your comment about the truth seems to reek of baloney. If you do not like the previous examples, then consider the 7.62mm x 39 cartridge. The europeans have a different idea of what constitutes a proper 7.62mm tube. We use a groove diameter, and hence, a bullet diameter of 0.308", give or takea little. The europeans use 0.310-0.311" for grooves and shoot bullets that are, nominally, the same size. Yet, we consume millions of rounds of foreign-manufactured ammo in our rifles chambered for this cartridge and all the American barrels are 0.308", or so. I don't hear of any big trouble from this one, either.

If you want, you can go to Krieger and order 30 caliber barrels with an amazine variety of bore and groove dimensions. No problems here, either, even with the exceptionally tight Palma barrels.

Many benchresters find that their rifles like to shoot "fatties." That is, bullets that are over the actual groove diameter. Don't seem to have any problems there, either.

Pushing snow in or out of a barrel is not an appropriate example for any argument outside of a soda fountain.

As a matter of fact, high powered rifles do have throats and lead bullets shot in those throats should be the same size as, or even 0.001" larger than, the throat diameter. The rhroat is, typically, 0.001" over groove diameter. The only problem here is the same as shooting lead bullets in an auto pistol, where oversize bullets occasionally cause feeding and chambering issues.
 

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Iowegan,

Here is another example that you might find interesting. You are fond of quoting names like QuickLOAD and so on. Sounds good, I'd say. So here is an expample.

I plugged in the following into QuickLOAD: .357 Magnum(SAAMI), Lyman 358156 bullet (of which I have shot many), and 13.5 grains of 2400 (a stiff load, no wimpy stuff here.

I calculated the pressure for a bullet diameter of 0.357" and then 0.360" and guess what? There wasn't enough difference in the pressure to worry about and it might even have been the same.

Go figure.
 

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pgeobc, Several years ago when I first bought QuickLOAD, I discovered several issues that bothered me so I called NECO and talked to one of the developers. This gentleman was very helpful although his European accent was a bit difficult to understand. The very first thing he told me was QL is not intended to be a reloading manual, rather an approximation of how different loads behave (I knew that). It is a great source for doing "what ifs" or to get a reasonably close approximation for chamber pressure, velocity, bullet travel time, etc. Because there can be so much variation from gun to gun, there's no way computer generated data can take every possible condition into account.

My first question concerned primers. There's no options for different brands or even changing from standard to magnum primers??? Answer: QL assumes a "Standard" primer for all listed data. Actual pressure tests indicate a very token change in chamber pressure between different brands of standard primers so there is no need to add primer brands to QL. For handguns, you can add .5 grains of powder to compensate for magnum primers. This will increase chamber pressure and velocity to approximately the same levels as when magnum primers are used.

My second question concerned pistols. QL velocity doesn't track very tight with reloading manuals or chronographs??? Answer: QL was designed for closed breach firearms such as a bolt action rifle. As such, there are losses affiliated with semi-autos where some of the pressure is consumed by slide/bolt thrust. This can not be calculated in QL because the weight of the slide/bolt and recoil spring tension are not the same in every pistol and directly affect velocity as does straight blowback versus delayed blowback. Most semi-auto pistols will display higher than actual velocity, however chamber pressure should be reasonably accurate.

Third question ... revolvers. There's no provision in QL to compensate for cylinder throat length, diameter, or B/C gap losses, plus changing bullet diameters doesn't seem to make any difference in chamber pressure???? Answer: Bullet diameter and cylinder throat diameters do indeed make a notable change in chamber pressure; however, because QL was designed for closed breach firearms, there's no way to compensate for them. If you change the bullet diameter, you will not see any notable change in chamber pressure. You can compensate for the other parameters unique to revolvers. First, measure the barrel in your revolver just like you would in a rifle or pistol ... from the muzzle to the recoil shield. As an example, a 38 Special with a 4" barrel will measure approximately 6". This will be QL's "barrel length". Second, the B/C gap will vent pressure and will reduce velocity by about 1.5% for each .001" of gap. Example: a normal revolver will have a B/C gap of .006". 6x1.5=9% loss so if the velocity at the adjusted barrel length (6" for a 4" barrel) is 1000 fps, the actual velocity will be about 1000x.91=910 fps.

I found by using the above recommendations, QL's calculated velocities track very close to the loads listed in Speer and Hornady reloading manuals plus they track quite well with my chronograph. From what the NECO gentleman told me ... if velocity tracks close to the same as loads found in a reloading manual, chamber pressure will also track accurately. I use QL mostly for calculating chamber pressure for lead bullet loads and as a "sanity test" for loads from a reloading manual. I realize QL is not a perfect resource and has some quirks, but it is close enough for my intended use.

Back to "bullets larger than throat diameters". I know for a fact this condition raises chamber pressure. How much? That will depend on the difference between the two and the caliber. A couple thousandths larger in a 45 cal bullet won't make as much difference in chamber pressure as the same discrepancy in a 357 cal. In other words, the smaller the caliber, the more chamber pressure will elevate with the same increase in bullet diameter. Obviously, the harder the oversized bullet, the more chamber pressure will increase. Again, because an oversized bullet is going to get sized down to throat diameter as it passes through, there is absolutely no advantage with using oversized bullets .... only disadvantages. As I mentioned in the above post, if you take advantage of chamber pressure and the proper bullet hardness and diameter, a bullet will expand (obturate) until it reaches the limiting size of the throats and will be delivered to the forcing cone about .001" larger than bore diameter. Using this technique, bullets seal very well in the throats and do not cause gas cutting. Although the same thing happens with oversized bullets, a proper sized bullet will not increase chamber pressure, nor will it cause endshake to increase, nor will it prevent a cartridge from fully chambering.

I didn't say your analogies were wrong about rifles ... they just don't have the same properties as a revolver .... oranges and apples. Maybe you should do some research in SAAMI's references. Although we say a typical 30 cal has a 308" bore and use .308" diameter bullets, SAAMI looks at it a bit different and rates bore diameter as the distance between the lands. In other words ... the largest pin gauge that will slide into a bore. For a 30 cal, that is normally between .296"~.298". Often times true bore diameter (distance between the grooves) in American made field grade 30 cal rifles is on the generous side ... up to .312. Match grade 30 cal rifles tend to be right on .308" between the grooves. What does this have to do with revolvers? Nothing.

When you run a gunsmith shop for 31 years like I did, you see the strangest things walk through the door. Revolvers with bulged barrels because the owner didn't take the time to remove excessive lead fouling. Shotguns with a football sized bulged barrel because the shooter had a squib then fired a round with a flimsy plastic wad in the barrel. Rifle barrels peeled back like a banana because snow or dirt got stuck in the bore. I see you didn't like my example but the point was ... it doesn't take much of an obstruction to cause a huge increase in pressure. Tight throats or oversized bullets aren't as destructive as a plugged bore but they are still an obstruction and do indeed increase chamber pressure.
 
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