Barrel/cylinder gap

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brutus51

brutus51

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Just out of curiosity I checked the gap of my revolvers. Always set my Dan Wesson at .004 thousands which works great, but my fixed barrel guns are as follows:

1978 Colt Python (My most accurate revolver) .006
Colt King Cobra Target .004
2022 Colt Python .005
Colt Cobra 38 Special .005
Colt Agent .003
Smith & Wesson 629 .004
Smith & Wesson 625 .45 Colt .005
Ruger single six 22lr. .004
Ruger single six 22 Mag. .004

Oddly enough the Colt Agent is an Aluminum frame parkerized tackle box gun.
Just found this interesting, kinda thing us old retirees have time for. :D
 
Your average is on the low end for production guns- the .006 to .009 range seems to be most common. Recently, the factories have let some extra breezy units to out the door well over .010.
 
.455_Hunter said:
Recently, the factories have let some extra breezy units to out the door well over .010.
Click to expand...
Ouch! I think .004 is perfect, my 78 Python is probably the most accurate because I've been shooting it for over 40 years, single action pull is somewhere around 11/2 to 2 lbs. Next most accurate is the Smith 6" 629 pre-lock but what has become my current most favorite is the 4" Cobra target, it's just the perfect size and balance. :cool:
 
For those drafty cylinders...

How long does one keep asking,

"...how does it shoot?"

I imagine that there'd be some safety concerns popping up.
 
My factory new Ruger LCR was 0.015". I haven't measured any others. This one let enough daylight through that I became suspicious and they fixed it right up. It worked fine of course and with a snubby I couldn't detect any accuracy issues. I like to think it works better now but I'll probably never know the difference.
 
Ideally a revolver should have a 0.004" to 0'008" barrel-cylinder gap, with 0.005" considered to be about perfect.

If you get below 0.004" you may start having problems with fouling build up on the rear of the barrel and front of the cylinder causing sticky cylinder rotation.
Get over 0.008" and you have some velocity loss, although to be fair, not all that much.
Within the last 10 years S&W is passing gaps of 0.012" as "in spec", and some have been seen over that.
 
When you measure that, do you pull the cylinder back or test it as it sits? Depending on the revolver, I can get .002 difference.

Something else I noticed is my feeler gauges are not accurate.
The 5 and 6 are spot on. 7 and 9 measure 8.5, 8 shows 7.5.
Measured with a certified Mitutoyo digital caliper I bought when I was building engines for a living.
 
The sootie cylinder on this new Charter Arms Pathfinder had a gap that measured .020"... it went back to the factory twice before they agreed to replace the barrel. It came back as .006" then I sold it. 3ABEB135-BBB2-4320-B58D-2F64391A509D.jpeg
 
In a semi-related question, does barrel twist have any measurable effect on velocity?
 
Those are all great numbers. The 4" 686 I sent to S&W performance center to be rebarreled to 7" came back to me with a .009" gap and a brand new Ruger New Vaquero .45 Colt I bought about 2018 has a gap so large I could drive a Mac truck through it. IIRC about .012"
 
Do you guys measure barrel gap with the cylinder pushed forward as far as possible or back as far a possible?
 
mcb said:
Do you guys measure barrel gap with the cylinder pushed forward as far as possible or back as far a possible?
Click to expand...
Neither. I measure B/C gap at rest. Forward and back measurements I use to measure endshake.
 
Last edited:
stonebuster said:
Neither. I measure B/C gap at rest. Forward and back measurements I use to measure endshake.
Click to expand...
How is that possible? Inserting the feeler gauge would push it back, would it not? I thought pushed forward was cylinder gap, and pushed back was cylinder gap plus end-shake? That is how I have always measured mine. Two numbers pushed forward, pushed back.
 
mcb said:
How is that possible? Inserting the feeler gauge would push it back, would it not? I thought pushed forward was cylinder gap, and pushed back was cylinder gap plus end-shake? That is how I have always measured mine. Two numbers pushed forward, pushed back.
Click to expand...
Your reply made me do some research where I found "true" b/c gap is cylinder pushed back and "apparent" gap is pushed forward. My method "apparently" measured neither.o_O
 
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eddiememphis said:
When you measure that, do you pull the cylinder back or test it as it sits? Depending on the revolver, I can get .002 difference.
Click to expand...

The proper way to measure the barrel/cylinder gap is with the cylinder pushed all the way forward. The amount the cylinder can slide back and forth is called End Shake.

According to Kuhnhausen B/C gap for a Colt Single Action Army should be .006 for jacketed bullets, .008 for lead bullets. End shake should approach 0. Endshake can be adjusted by adding shims under the cylinder bushing, which will have the effect of keeping the B/C gap to the largest dimension.

Kuhnhausen says Ruger single action revolvers should have a B/C gap of .004 - .006 for jacketed bullets, .008 for lead bullets.

Revolvers without a top strap often develop excessive barrel/cylinder gaps over time as the frame tends to stretch a little bit. This antique Merwin Hulbert Open Top Pocket Army has a B/C gap of about .010 with about .008 end shake.

plcAM7Ijj.jpg




It really does not matter much though.

plz5ozxOj.jpg
 
B/C gap, in the way we all are concerned by it (bullet velocity and fouling), is measured with the cylinder pushed all the way to the back, always. That is because as soon as the bullet leaves the cylinder, the expanding gasses push the cylinder towards the shooter, away from the forcing cone and only after this (bullet leaving the cylinder) the B/C gap comes to play. What happens before that is of no concern - i.e. bullet still in cylinder, when (only in cartridge revolvers, mind you!) the cylinder is pushed forward.

Barrel to cylinder gap is measured for three reasons and right there lies the confusion with it:
1. Determining endshake - measuring B/C gap with cylinder fully forward, then fully to the rear. The difference between the two measurements is the endshake. Mind you, we are not concerned at all about B/C gap here, we are only using those measurements to determine completely different tolerances - if endshake is in spec, nothing more.
2. Determine if there is a cylinder drag on the barrel, or insufficient clearance between barrel and cylinder - B/C measured with cylinder fully forward.
3. "Real" B/C gap, the one that affects velocity of a given projectile - cylinder fully to the rear.

So, to measure the B/C gap only with cylinder pushed forward, to the muzzle, does nothing to tell us what it will be when shooting the said revolver. If that was the case, then all of the Colt C&B revolvers, including clones, will have ZERO B/C gaps - i.e. none.
 
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Just measured all S&Ws at around .005" including the 686-5 I just had the factory do a barrel replacement on. This include a number of 686's, a couple of 629's and a 929. Forgot to measure the 100yo Lemon Squeezer, but I think it's very tight as well.
 
Adhering to the time honored way of measuring for gap and endshake, I have :

Two Dragoons (45C cart. guns) with 0 gap. One has .0015" and the other has a snug .002" endshake.

Two '60 Army's ( one 45C. one 45acp) with 0 gap. The acp has .015" , the 45C has .002" endshake.

A '58 Rem.(P) in 45C ( Kirst gated) with .003" gap, .002" endshake.

A Cim.Frontier (P) in 45C with .005" gap, and 0 endshake. Same with 45acp cyl. has .007" gap, with 0 endshake.

4 ROA'S with Kirst cyls. (45C) all over the place!!
Gaps and endshake at .003" - .0035", .004" - .005", 0, - .0085 and .0085", - 0. !!!

Mike
 
Mizar said:
B/C gap, in the way we all are concerned by it (bullet velocity and fouling), is measured with the cylinder pushed all the way to the back, always. That is because as soon as the bullet leaves the cylinder, the expanding gasses push the cylinder towards the shooter, away from the forcing cone and only after this (bullet leaving the cylinder) the B/C gap comes to play. What happens before that is of no concern - i.e. bullet still in cylinder, when (only in cartridge revolvers, mind you!) the cylinder is pushed forward.

Barrel to cylinder gap is measured for three reasons and right there lies the confusion with it:
1. Determining endshake - measuring B/C gap with cylinder fully forward, then fully to the rear. The difference between the two measurements is the endshake. Mind you, we are not concerned at all about B/C gap here, we are only using those measurements to determine completely different tolerances - if endshake is in spec, nothing more.
2. Determine if there is a cylinder drag on the barrel, or insufficient clearance between barrel and cylinder - B/C measured with cylinder fully forward.
3. "Real" B/C gap, the one that affects velocity of a given projectile - cylinder fully to the rear.

So, to measure the B/C gap only with cylinder pushed forward, to the muzzle, does nothing to tell us what it will be when shooting the said revolver. If that was the case, then all of the Colt C&B revolvers, including clones, will have ZERO B/C gaps - i.e. none.
Click to expand...

This. Endshake contributes to b/c gap, but can't be excluded from factoring into its measurement.
 
LaneP said:
This. Endshake contributes to b/c gap, but can't be excluded from factoring into its measurement.
Click to expand...

And here is where the difference is between open top revolvers (Colt type) and most top strap framed revolvers. Endshake is destructive!! With "0" gap and all endshake, the open top can be closed up to a .002" - .003" endshake which increases efficiency, decreases fouling (exterior and in the barrel) any build up of fouling on the cyl face gets disturbed (wiped) with each cycle of the action. This is why I describe them as "self cleaning ". That amount of endshake is too small to be detrimental to the setup and will be maintained. The smokeless setups can handle .0015" - .002" (max) with the same results. Smokeless is much more punishing than bp.

For top strap revolvers, the endshake can be shimmed away (if you have a removable bushing) or stretched. Doing that adds to the gap side of things but it removes the "detrimental" aspect of endshake. As for gap, a barrel setback is in your future (or possibly fitting a longer cylinder if you're lucky!!).

Mike
 
45 Dragoon said:
...Doing that adds to the gap side of things...
Click to expand...
No, it doesn't - the B/C gap reaches it's maximum dimension as soon as the bullet leaves the cylinder. Every other dimension is purely "cosmetic" if we are discussing B/C gap relations to velocity, fouling and powder spitting. Endshake is a completely different dimension, not connected directly to the gap. Take a very close look at this video - start from 0.30 min.
 
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