bc gap
- Thread starter gdog357
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What is your barrel/cylinder gap currently measuring and what do you want to get it to?
The barrel needs to be removed, machined and then reinstalled. This sets back the barrel by one additional turn.
Then, if needed, the back side of the barrel can be refaced or reduced to adjust the B/C Gap
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Alk8944
US Veteran
.010" is within the current S&W specification of .012" or less, don't mess with it!
I once reduced the B-C gap of a J-Frame Model 631 that had a gap of .012" as I recall, to under .002". The gun was chronographed with the same ammunition both before and after this and there was exactly no increase in velocity! The B-C gap has no effect whatsoever on accuracy. What did you hope to accomplish??? Whatever it is you will not see any improvement worth the time or cost.
If you expect a velocity increase just add 1/10 grain of powder to your charge and you will get more than by reducing the B-C gap!
Note that when I chronograph ammunition for average velocity I shoot a minimum sample of 10 rounds, sometimes up to 50 rounds, instead of the 3-5 rounds that many do. This provides a statistically significant result.
I once reduced the B-C gap of a J-Frame Model 631 that had a gap of .012" as I recall, to under .002". The gun was chronographed with the same ammunition both before and after this and there was exactly no increase in velocity! The B-C gap has no effect whatsoever on accuracy. What did you hope to accomplish??? Whatever it is you will not see any improvement worth the time or cost.
If you expect a velocity increase just add 1/10 grain of powder to your charge and you will get more than by reducing the B-C gap!Note that when I chronograph ammunition for average velocity I shoot a minimum sample of 10 rounds, sometimes up to 50 rounds, instead of the 3-5 rounds that many do. This provides a statistically significant result.
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Some of us are more picky than others. When I look at a used S&W, or any other brand, the BC gap is one of the things I look at. If it looks to be a bit too large I don't buy the gun.
Leave it alone. It is within S&W's specification and a smaller gap would result in remarkably little velocity gain. A minimum gap can result in cylinder bind when there is a build up of residue, so I'd rather have a larger gap than a tiny gap.
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Yup - according to the new S&W specs, the max is .012". Used to be .006" but that was when they made them right IMO. Because it falls within their current spec, they will not fix it. As long as you are getting accuracy and do not have any lead spitting, your only consequence will be a slight bit more velocity loss.
To make the B/C gap smaller, the barrel would have to come off and have its shoulder cut to line up exactly correctly. Then the forcing cone would need to be filed down to a smaller B/C gap size.
To make the B/C gap smaller, the barrel would have to come off and have its shoulder cut to line up exactly correctly. Then the forcing cone would need to be filed down to a smaller B/C gap size.
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and the ejector rod and center pin shortened .0277, which is how much the barrel sets back when it is threaded in one full turn
But I doubt that there is even a 20fps difference in velocity from a .006 gap and a .010 gap
But I doubt that there is even a 20fps difference in velocity from a .006 gap and a .010 gap
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A new spec of .012 seems to me a reduction in the quality of new revolvers.
Protocall_Design
Vendor
That's only because it is.
The only time I have every messed with the B/C gap was if it was a functional issue. If it works, it isn't spitting at you or having other b/c gap type problems I would not bother. If it bothers you you can always pay to have a competent gunsmith deal with it, but is it worth the time and (probably modest) amount of money for you to deal with it? No wrong answer there.
JRD
Member
Where are you guys hearing the new S&W factory spec is .012 for max B/C gap?.
My sources at the S&W factory say they are still working to .010 max.
My sources at the S&W factory say they are still working to .010 max.
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My most recent manufacture Smith revolvers are a 27-9 from 2012 and a 36-10 from 2015. Both of those have b/c gaps of .006. Perhaps I was lucky. My Colt Python bought new earlier this year gauges at .004.
A combination of assembled parts that produces a B/C gap of .012 and under signifies about the best we are gonna get. Nothing to fret about as you will never know the difference unless end shake develops. Get the B/C too tight and it will likely rub on the cylinder somewhere or rub when it gets hot.
Piwo
Member
I have a model 49 bodyguard I bought in 1989. The b/c was .018!!! I sent it back to the mothership and it returned with a gap of .011. I just decided to live with it as accuracy was unaffected. That was in about 1994 or so.
Jeff423
US Veteran
FWIW: I set the BC gap at 0.005-0.006 on my Dan Wesson .357 and it seems to work well.
The actual complete correct answer. LOL
Remove the barrel and either use a special tool or a lathe setup like mine to remove .0277 off the back shoulder and .02 off the back face of barrel extension. (This will close up gap .0077) Check the forcing cone and adjust if needs touched up a bit. If it is a pinned barrel, file the notch for pin in the barrels threads forward 1 thread. Re install barrel and pin. Gently start to close cylinder and observe how ejector rod hits the barrel lug tooth. Remove cylinder. File or in my case use a fine belt on a industrial belt grinder to shorten rod so it will go past tooth. Then hold a piece of flat bar against the center pin where it sticks out of ratchet so tip is held flush to ratchet and remove excess center pin sticking out the tip of ejector rod. You want it to just slightly clear the end of rod. Put slight bevel back on OD of rod and then twirl a small drill bit held in your fingers against the tip of the ejector rod to put a tiny bevel in the hole. Close cylinder and check gap, If to tight, adjust with a file that has sides safed. Hold file dead flat on face of barrel extension as you make slow passes until you get the exact clearance you want. I put a small piece of stainless foil between top of barrel extension and frame to protect the finish there, but gap blast will quickly fix that anyway if you actually shoot your gun much
Been there done that. Old K38 barrel setup for photo
All this will gain you a minimal increase in velocity which will not effect making holes in paper or in the very worse case, cost you an 1/8" of penetration in a soft target.
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PS
Set up like this with the indicator at 45 degrees to the surface you will need to read a - 1.4142 times the .0277 (.0392) to actually remove the .0277. I get close, then try barrel on frame, and it it tightens up to soon go back in lathe and run a file on shoulder face. 10 degrees only takes .007
You can also set up using a 4 jaw chuck, but this way the barrel's muzzle and forcing cones are turning on their centers. The brass piece at live center has a blind 1/4 20 hole and the jaw end one goes all the way though. I made another set using 10-24 all thread when I fit a 22 jet barrel to a model 18 frame and needed to remove almost 1/10" of the back of ejector rod shroud and barrel's shoulder to get enough extension to close up the gap for a regular length K22 cylinder
Here is the result with an spare cylinder in 22 mag
Set up like this with the indicator at 45 degrees to the surface you will need to read a - 1.4142 times the .0277 (.0392) to actually remove the .0277. I get close, then try barrel on frame, and it it tightens up to soon go back in lathe and run a file on shoulder face. 10 degrees only takes .007
You can also set up using a 4 jaw chuck, but this way the barrel's muzzle and forcing cones are turning on their centers. The brass piece at live center has a blind 1/4 20 hole and the jaw end one goes all the way though. I made another set using 10-24 all thread when I fit a 22 jet barrel to a model 18 frame and needed to remove almost 1/10" of the back of ejector rod shroud and barrel's shoulder to get enough extension to close up the gap for a regular length K22 cylinder
Here is the result with an spare cylinder in 22 mag
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