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Discussion Starter #1
I read Old No 7's post on recutting the forcing cones and chamber mouths on a pair of Ruger D.A. revolters. He stated that he used an 11 degree cutter on the cones. I was under the impression that the cones on Ruger D.A. handguns are factory cut at 5 degrees, read that in brownells catalog in the reamer section. Am I misinformed? I know the D.A. cones are indeed cut "gentler" than the cones on the S.A. guns.
In ransom rest tests that I have done, the D.A. wheelguns almost always shade the S.A. guns in group size, sometimes by a wide margin with both jacketed and cast bullets. I understand there are more issues at play here than just cone angle such as cylinder/barrel alignment, bore condition and so on but it stands to reason that the gentler the entry angle ,the less the bullet will be battered while trying to align itself on its trip down the tube.

Longpoint
 

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I was surprized about 11 deg angle too. Here is a close up of the forcing cone of my GP-100. It appears much steeper then 11 degree.



Mike
 

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LongPoint, For many years my "fix" for revolver accuracy has been to ream the throats of the cylinder to .0015" larger than bore diameter and to chamfer the forcing cone with an 11 degree reamer. I've yet to see this combination fail. In most cases, accuracy and fouling are dramatically improved. I've cut several hundred forcing cones in my day ... most in S&W DA revolvers, some in both SA and DA Ruger revolvers .... I have seen a few guns where improvements were minimal but never worse than before chamfering.

If cylinder-to-bore alignment was perfect, you wouldn't need a forcing cone at all. We all know perfect alignment is not one of Ruger's attributes so you need a slight taper to get the bullet started into the bore without distorting it too much. A more gentle funnel shape will guide the bullet into the bore with less shaving and makes for a more accurate revolver. Factory cut forcing cones from Ruger are very often corrupt with machine marks, divots, and rough spots. hoptob's photo is perfect evidence. Once the cone is cut smooth from the mouth to the lands, fouling will be minimal and accuracy will improve. When you cut a new forcing cone, the angle must be greater than the previous angle or you will end up with a compound cut (multi-angle) which is very bad for fouling and accuracy. Ruger cuts their cones from 5 to 8 degrees so a 11 degree cut works quite well in all revolvers from a 38 to a 45 cal.
 

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Discussion Starter #4
Iowegan, I'm with you on the gentler angle reducing bullet deformation. If the angle is measured from bore centerline wouldn't 5 degrees be a longer angle than 11 degrees or am I looking at this bassackwards? I've been casting my own bullets going on 25 years and size .001" over groove diameter and open chamber mouths so bullets are a snug push fit. When I set a bullet base first in the forcing cone of my GP and then mark the bullet at the breech end and take a measurement, the bullet base contacts the rifling leade at .330", do the same thing with one of my Blackhawks of same caliber the bullet makes contact at .195". The GP has a more bullet friendly angle.

LongPoint
 

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Thank you, Iowegan!

LongPoint, the way I understood it is that 11 deg cut is steeper then 5 deg cut (as you pointed out). But both are equally long - they have to start at the mouth and end at the lands. 11 deg cut will open mouse more and cover all of the 5 deg cut area. This way you will not end up with a compound angle.

Mike
 

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Discussion Starter #6
hoptob and Iowegan, I'm still with you guys, but seems to me that opening up the mouth of the cone would only be needed if the piece were spitting lead from the barrel/cylinder gap. In the interest of leaving as much meat as possible in the forcing cone mouth area and still cleaning up the cone to take care of tool marks or an off center cut cone the shallower cutter would be still prefered. It would probably take more than a few lifetimes to put enough rounds through a Ruger to split or erode the forcing cone enough to matter. The top strap would probably give up before the cone, but still I would think more is better and the gentler angle would be even more cast bullet friendly.

LongPoint
 

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hoptob has it right. Reamer angles do get confusing. Think of it this way ... no cone at all would be zero degrees. As the angle of the cone widens, the numbers get bigger. An 11 degree forcing cone will be cut to about the same depth but will be wider at the mouth than either a 5 or 8 degree cone.

LongPoint, I'm not sure your method of measurement has anything to do with a "more friendly bullet angle" but it probably does indicate the two cones were either cut at different angles or at different depths. Cones can be very misleading ... especially on Ruger revolvers. I've seen some barrels with a very slight cone, almost none at all and another gun in the same model and caliber where the cone was cut quite deep. If you use a plug gauge, you can actually measure cone depth but it is really a meaningless measurement.

The two primary reasons to chamfer a forcing cone are both related to accuracy. If the factory cone is corrupted, then there is a good chance accuracy and fouling will improve (they always go hand in hand) when the cone is reshaped and smooth. If the cylinder-to-bore alignment is a tad off (very typical in any production revolver) then a wider angle forcing cone helps direct the bullet into the bore with minimal distortion and again, accuracy and fouling are improved. If your revolver bench shoots 1" or better groups at 25 yards, you're wasting your time chamfering the cone.

There is one downside to a wider forcing cone angle and that is velocity loss. A typical 357 Mag with an 11 degree forcing cone will loose about 25 fps more to gap spitting than a 5 degree cone. This is a very token loss and in my opinion, well worth sacrificing for better accuracy and less fouling.
 

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Discussion Starter #8
Iowegan, I think we are both dancing around the same bush here. Lets take two theoretically identical barrels. Same bore and groove diameters.

Barrel #1 has a forcing cone cut with a 5 degree taper. It is cut so there is a mouth diameter of say .375" to allow for all the misallignments and so forth.

Barrel #2 has a forcing cone cut with a 11 degree taper. It is cut so there is a mouth diameter of the same .375" for the same reason.

On barrel #1 the cut will extend farther up the barrel resulting in a gentler entry ramp for the bullet, resulting in less deformation.

LongPoint
 

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Well, if one can start with uncut barrel, then I suppose 5 deg may a way to go. But what if you are trying to correct rough factory cone which is already cut at 5 deg? Then you have to use steeper angle (11 deg) and cut to same or greater depth than factory cone. One can't correct 5 deg cut with another 5 deg cut. Right?

Mike
 

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LongPoint, What you said is absolutely true but that would require a pretty deep forcing cone to make the mouth large enough to allow for cylinder misalignment. From a practical point, it's much easier to gently ream the cone with an 11 degree tool and end up with a smooth transition that works well in any caliber.

Because all revolver barrels are reamed with a tapered tool, you could indeed have the same diameter mouth with different angles. The more narrow the angle (closer to zero degrees) the deeper the forcing cone will have to be to allow for C-T-B misalignment. Herein lies the problem like hoptob said ..... if you start with a factory 5 degree cone, you have to chamfer it with a 8 or 11 degree reamer because a 5 degree reamer would end up going way too deep by the time you dressed the bad spots. You can never ream a larger angle cone with a narrow reamer (example: 8 degree cone ... 5 degree reamer) because you will end up with a compound cut. So ... to minimize the expensive tools, most gunsmiths buy just the 11 degree reamer and use it on any barrel from .357" to .451".

This subject is so confusing that even Brownell's instructions have it wrong. They tell you to ream with a smaller angle to avoid compound cuts. Anyone that got a D or better in geometry class should know this isn't true.
 

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Discussion Starter #11
Good morning Fellas.
Hoptob, you can correct a beat up 5 degree cut with a 5 degree reamer, you will simply be taking metal off the entire cut from the mouth to the top of the lands and widening the mouth only slightly in the process.
You can also correct a beat up 5 degree cut with an 11 degree reamer but you will be removing metal starting at the mouth and progressing down the bore. If the reamer is advanced far enough to clean up the cut to the top of the lands [ as it should be to avoid that coumpound angle] it will have hogged out the mouth a good bit more than a lesser degree cutter would have.
Iowegan, maybe I went fishing that day instead of school.

LongPoint
 
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