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Discussion Starter #1 (Edited)
Looking around RF, and other places, I get fifty different ways to check the barrel/cylinder gap on a revolver. With the cylinder static, pushed forward, pushed back. With and without cartridges in the chambers. Etc., etc.. I think some people only do it during a certain moon phase.....:rolleyes:

I've always understood that with an empty cylinder pushed forward, that was the B/C gap. Pushed back was headspace, and the difference was end shake, or cylinder play. Or maybe I've read so much, I'm confusing them now?

My four revolvers are such:

Bearcat one, .003 forward, .005 back, endshake of .002.

Bearcat two, .005 forward, .007 back, endshake .002.

LCR-357, .004 forward, .008 back, .004 endshake.

LCR-22, .003 forward, .008 back, endshake .005.

I had a Single-Six that measured .003 and .007 after Ruger re-barreled it to fix a .012 B/C gap. They knew I wanted a tight gap, so that should have been the .003 with the cylinder pushed forward? I understand Ruger's standard is .004-.007.

And with countersunk rim chambers like the Bearcat, I don't think the measurement is right with the cylinder pulled back, since the cylinder has a raised rim on the back that contacts the recoil shield?
 

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bearcatter, The most accurate way to express B/C gap is exactly how you listed your four revolvers. If endshake was zero, just one measurement between the rear of the barrel and the front of the cylinder face would suffice. Unless headspace is grossly out of spec, it makes no difference if cartridge cases are chambered or not.

"True" B/C gap is when the cylinder is held firmly to the rear. This places the cylinder where the ratchet column is pushed tightly against the recoil shield. It is also the position of the cylinder when the gun fires and the base of the bullet crosses the B/C gap and allows pressure to vent. If you install endshake bearings in a DA revolver or a shim between the end of the gas ring and frame on a SA revolver to reduce or eliminate endshake ... it will push the cylinder back until the ratchet column contacts the recoil shield. This will result in the widest B/C gap measurement (true B/C gap). With 22 cal revolvers "true" B/C gap shouldn't change in the life of the revolver. With centerfire and especially magnum revolvers that have been fired a lot, the ratchet column will peen and the recoil shield will develop a "divot" resulting in a wider "true" B/C gap.

"Apparent" B/C gap is when the cylinder is pushed forward. This "number" is used for two purposes ... to determine endshake and to determine if the barrel is so close to the cylinder face that it will rub when the action is operated. With 22s, apparent B/C gap probably won't change at all for the life of the revolver. With centerfire revolvers that have much higher cylinder thrust, apparent B/C gap will get tighter as endshake increases. The more powerful the cartridge, the faster endshake increases, and the more it increases, the more space it has to develop more momentum and increase even faster. Using bullets that are larger than throat diameter will also cause endshake to increase faster.

On your Bearcats ... a couple thousandths endshake is desirable so you can remove/replace the cylinder without using a shoehorn. For DA revolvers, a few thousandths endshake is also desirable to keep the cylinder from locking up when corrupted by powder residue. The "magic" measurement where endshake can start getting out of control in a centerfire revolver is when it exceeds .005". Think of it like driving a nail with a hammer ... the longer the hammer throw, the more momentum you will develop. With a long hammer throw, you may be able to drive the nail in with just a few blows, whereas if you only have a few inches of hammer throw, it may take several dozen blows to drive the nail in. Cylinder endshake works exactly the same way ... more power coupled with more cylinder movement will cause endshake to increase.

On to "headspace" ... by definition, headspace is the distance from the cartridge head to the recoil shield or bolt face. That means you must have a cartridge chambered under the firing pin to get a measurement by placing a feeler gauge blade between the two. Just like B/C gap ... there are two headspace measurements ... one with the cylinder held back, which would be "true" headspace and the other with the cylinder held forward ... which is "apparent" headspace. Apparent headspace is endshake dependent and is one of the leading causes of misfires (light primer strikes). The thickness of the ratchet column regulates "true" headspace so when the cylinder is held back, headspace will be at its minimum. With centerfire revolvers, the normal "true" headspace is .010" +or- .002" (.008~.012"). This will allow enough space between the cartridge head and the recoil shield so case heads won't bind up due to variations in rim thickness. It also allows a little extra room for "high primers". Because rimfire cases don't have primers, the normal headspace is .006" +or- .002" (.004~.008"). Again with 22 cal revolvers, true headspace will rarely change for the life of the revolver. With centerfire revolvers, the ratchet column and recoil shield will peen as more and more rounds are fired. This effectively shortens the ratchet column making "true" headspace get tighter as the gun wears. "True" headspace is rarely an issue ... it's "apparent headspace" that causes problems. First, it will increase endshake and second, it will allow the cylinder to move forward when the hammer smacks the firing pin, thus cushioning the blow. This will cause light primer strikes (or rim strikes on 22s) and may result in a "click-no bang" condition.

To understand how these above conditions affect the function of a revolver, it helps to know what happens when a round is fired. In a centerfire revolver, starting when the hammer strikes the firing pin ... the primer detonates and because the flash hole is so small, pressure from the primer flash will cause the primer back out until it contacts the recoil shield. As powder ignites and chamber pressure builds, the cartridge head is thrust against the recoil shield, which will reseat the primer. At the same time, chamber pressure starts pushing the bullet forward. When a bullet passes through the cylinder throat, friction between the bullet and throat will force the cylinder to move forward with the bullet under thousands of pounds of pressure. The larger the bullet, the more friction and the more violent forward cylinder thrust will be. The cylinder will stop forward movement when it contacts the limit. In a DA revolver, the limit is the crane tube end contacting the boss inside the cylinder's center hole. For a SA revolver, the limit is when the cylinder's front gas ring contacts the frame.

When the base of the bullet passes the B/C gap, chamber pressure will be very high, which will push the cylinder to rear until the ratchet column strikes the recoil shield. It is at this point where the B/C gap is the widest allowing the maximum amount of pressure to vent. Forward cylinder thrust is far more violent than rearward cylinder thrust so this is why most of the endshake issues come from peened crane tubes in DAs or peened front frames in SAs. Rimfire revolvers work much the same but because there is no actual primer, the first step does not apply. Also because of much lighter bullets and way less "power', rimfire DA and SA revolvers rarely suffer from excessive endshake ... unless they left the factory that way.

To the best of my knowledge. Ruger has not made their B/C gap, headspace, and endshake specifications public. If they did, they would likely have more guns returned for repairs from us "perfectionists". From what I have seen, Ruger doesn't get too excited until B/C gap exceeds .010".

Just another FYI ... on Single-Sixes and Bearcats with recessed chambers, it is not unusual for the outer rim of the cylinder to touch the recoil shield ... it shouldn't but sometimes it does. This makes it very hard to measure headspace. Usually you can open the loading gate to access the headspace gap but you will need a narrow feeler gauge blade. Personally, I wouldn't worry about headspace on a 22 unless you find the cylinder is binding on the case rims.
 

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Discussion Starter #3
Thanks for the lesson, Professor, and I mean that in an amazed way. So, my whole life I've considered "apparent" B/C gap to be the B/C gap.

Both of my LCRs seem to have a large "true" B/C gap, but others I've looked at are the same way. I'm supposing it has to do with the LCR design?

This confuses me also about people that want a "tight" revolver with a .002 B/C gap. If this were the "true" gap that you describe, the gun couldn't practically have more than .001 endshake, or the cylinder could rub the forcing cone. That would give the gun a .001 "apparent" B/C gap, with the cylinder forward.

An "apparent" B/C gap of .002 could go with varying "true" measurements and endshakes.

I think I'm confusing myself again........:D
 

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bearcatter, B/C gap is way over rated ... almost as bad as a bank vault solid cylinder lock up. Yes, B/C gaps rob a little velocity because pressure vents ... but that is just the nature of a revolver and is really no different than slide or bolt thrust robbing a little velocity from an autoloader. B/C gap has no significant influence on accuracy in any revolver.

In a perfect world, a revolver would have zero endshake and just enough B/C gap to keep the cylinder from rubbing on the barrel. Good luck finding a revolver like that ... maybe a high $$$ custom made gun but certainly not a factory made gun. And even if you could find the "perfect" revolver, chances are the cylinder would bind up with powder and bullet residue after the first few rounds were fired. So basically, a gun with a tight B/C gap is good for braggin' rights ... nothing more. I like to see a True .006" B/C gap ... nothing less than .004" because fouling will build up on the cylinder face and cause the cylinder to bind on the barrel. A .006" B/C gap pretty much guarantees a "no bind" condition even with lead bullets and normal endshake.

Endshake is a different story because it is a sign of wear (or factory misfit). As endshake increases with use, the chances of a weak primer hit also increase. If endshake is allowed to get out of control, the cylinder lock notches will release from the cylinder latch when fired. With an unlatched cylinder, it's hard to say where the bullet will go ... maybe in the forcing cone, maybe strike the edge of the barrel, maybe the frame. Anything but a good cylinder-to-bore alignment will surly cause major problems. True B/C gap will track with endshake and get wider with use.

Here's a bet for you concerning your Bearcats and LCR-22 ... record your B/C gap and endshake specs as noted above. 20 years from now and many bricks of ammo later, measure them again. My bet ... there will be no significant change in either B/C gap or endshake and all three guns will still be within specs.

LCR-357s haven't been on the market long enough to establish a track record so I wouldn't take any bets on yours. If it follows the trend with other 357 Mag snubbies as I suspect it will, endshake will likely increase gradually until it gets more than .005". How many rounds that takes will depend on how hot the loads are. With any 38 Special loads, it will take a very long time. With a steady diet of 357 Magnums ... not very long. So ... save the magnum ammo for social events, practice with 38s, and your LCR-357 will last a long time before repairs are needed.
 

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Discussion Starter #5
Thanks again, and I'm sure others have benefitted from your expertise here.

I have no doubt of insignificant change in any .22LR revolver. My LCR-357 will probably never chamber a .357, so it will outlast me by a good margin. By my usual shooting habits, it won't fire more than a few hundred .38s a year .

I've always found the technical aspects of gun design interesting, and think it makes me a better shooter.
 

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To the best of my knowledge. Ruger has not made their B/C gap, headspace, and endshake specifications public. If they did, they would likely have more guns returned for repairs from us "perfectionists". From what I have seen, Ruger doesn't get too excited until B/C gap exceeds .010".

Excellent write up, Iowegan.

Just FYI, I did get those measurements this morning when I called. No problems getting them, but the headspace made little sense to me.

However, the stats I list below are for a Super Blackhawk, so it isn't necessarily useful in THIS application.

Cylinder Gap: .003-.010" is 'acceptable'.
Endplay/Endshake: .003" is Maximum allowed.
Headspace: .060-.066" is the 'acceptable' range.

My confusion is that my measurements of headspace came up with measurements of:

True Headspace: .002"
Apparent Headspace: .010"

Yep, I've got about .007-.008 Endshake/Endplay too.

But, what threw me off was their measurement of .060-.066" compared to my .002-.010" range.

I would understand .006-.0066" better, but when I asked if it was .006, they said .06, I believe. Maybe I didn't hear just right.
 

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bczrx, Ruger measures headspace a bit different than most gunsmiths. They measure the distance between the rear cylinder face and the recoil shield without a cartridge chambered. In other words, when you add the thickness of the case rim (.060") to actual headspace (.006"), you will get .066".
 

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Ahhh,

thanks!
 
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