question about shimming cylinder gap on j frame

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labnoti

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Working on .38 +P handloads for j-frame (642/442), and seeing less velocity than wanted. It's probably just the usual penalty for a short barrel, but I like to find solutions.

I'm new to revolvers so if I appear clueless, that's why, just clue me in.

J frame is new from factory, has about 300 factory (Federal American Eagle) rounds through it. It was a web order through a big box store so it wasn't fiddled with at the gun counter.

Measured the cylinder gap with feeler gauges and see it's 0.013" on the left side, and 0.010" on the right. I measured all 5 chambers and they're about the same, so there's no wobble or unsquareness in the cylinder face. It looks like the forcing cone is cut at a slight angle. I can measure the edge of the cone to the face of the frame is at least a thou shorter on the left side. Is it cut that way to keep a cylinder with some end shake from hitting the side of the forcing cone?

The end shake is slight. The cylinder does move, but it's probably a thou or two.

All in all, I figure it's within S&W spec for the model. There's nothing that strikes me as way off considering the price point and that this isn't a custom job. But it seems like a tighter gap would cost less pressure and velocity.

I had a question about shimming, but I've since come to understand from reading that the shims will reduce end-shake, but closing the gap any beyond that will require screwing the barrel in farther. It seems at best I could tighten up the end-shake 0.002", and that wouldn't be worth it.
 
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Im no smith, but I think your asking about pushing the cylinder forward with a shim to tighten your gap? If so, my immediate thought concerns the excess space you'll create behind the cylinder. Your firing pin would likely not protrude far enough to ignite primers. I suppose it all depends on how much shim you’re adding, might could get away with .02, but like I said, i’m not a gunsmith. I think barrel set back is the only way to properly achieve what you want.
 
The cylinder barrel should be consistently need side to the other and .013 seems excessive..actually so does .010 but that may be within factory specs, though just barely
As The above post says, if you shim the cylinder forward you risk light strikes due to excessive head space depending on where you already are in relation to tolerances
 
This question is best asked on the gunsmithing forum.

I think maximum b/c factory spec is .012". I personally like .008" on my competition and self-defense revolvers. Truing up the rear of barrel and cutting a forcing cone is a quick, easy job for a gunsmith with the right tool. That will leave you with a .013" gap. IMO it doesn't matter on a revolver such as this as most of your velocity loss is from the short barrel.

You might give S&W a call and find out what current allowable specs are.
 
Chances are the factory will say the gun is with in specs. If you want a closer barrel cylinder gap, then a trip to your
gunsmith is in order. He will reset the barrel, one turn in, and reface the end of the barrel, setting the gap.
If he's good the gap will be less then 4 thousands. Some people might say you will get a cylinder bind when the gun heats up, but
your looking at shooting just one cylinder full when needed.

I have had a Colt replica barrel reset (.003 gap) in 45 Colt have had no problems with binding.
 
What kind of velocity are you getting? Closing the gap may give a small gain but if you are looking for 75fps you aren't going to get it closing the gap by .002
 
Endshake shims reduce forward movement of the cylinder, increasing B/C gap. The only conventional ways to reduce B/C gap are to set the barrel back a turn, or fit an overlength cylinder. Neither is a practical solution here.

Shoot it, and be happy. If you shoot enough cast bullets, the vapor deposition on the cylinder face will reduce B/C gap.
 
Jonesy814 said:
What kind of velocity are you getting? Closing the gap may give a small gain but if you are looking for 75fps you aren't going to get it closing the gap by .002
Click to expand...

I don't know since I haven't put it over the screens with the larger gap.
My b/c gap was .012, the reduced blast from b/c was well worth the cost of setting back the barrel.
 
Facing off the yoke will increase the endshake - say, you want to reduce the B/C gap from .012 to .004. That will introduce .008 play in the cylinder which some will consider excessive. Add that to the already existing endshake. On top of that, the B/C gap will stay the same as before the intervention - at the moment the bullet clears the cylinder the expanding gasses will push the cylinder back to the breechface when escaping via the B/C gap, leaving you with the same gap you started with - .012. The only proper way to deal with that situation is to remove the barrel, face it one turn, screw it back, adjust the B/C gap and re-cut the forcing cone. The problem here is that OP has an alloy framed revolver which many gunsmiths refuse to take the barrel off from fear of cracking the frame.
 
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I'm with the others who feel any velocity gains from tightening the CG would be insignificant at best, especially from such a short barreled gun. I too would be more concerned with accuracy and reliability than another 10-20fps.
 
Just getting into revolvers and .013 seems a bit much on a new gun. Researching cylinder lockup the figures of .002-006 as being acceptable are bandied about, and .013 is double that. As you get down to the tighter spec then heat expansion is possible, but more likely an extended session of firing would have powder residue build up and that seems to cause the bigger issue requiring a simple wipe down to relieie.

Tighter gaps do increase gas down the barrel and some claim they can document a certain amount of gain/loss in terms of fps. As said the bigger issue is that a 2" barrel doesn't get much dwell time and makers are adjusting by offering loads tailored to snub noses to regain it. Given the factory is usually excoriated for tighter gaps with cylinder lockup rather than a negligible loss of speed for a close range defense gun you see where they put their emphasis.
 
I think the b/c gap is a function of mfgr's quality control dept. A gun with that kind of gap should have never left the factory.
I have several revolvers from different makers and none of the new guns came with that kind of gap.
As I have said before my SA had a .012 gap, but that gun is over 22 years old and could have been used for CAS,
Even my newest(new to me) Super Redhawk does not that kind of gap and it is a first year issue of the Super RH, and chances are it has seen more
heavy loadings than many other "service type" revolver's.
 
Howdy

I checked my Kuhnhausen book and he specified barrel/cylinder gap should be about .004 to .006. This is with the cylinder pushed all the way forward.

I grabbed a couple of older J frames and measured them

A Model 36 from 1961 is measuring a gap of .002 with an endshake of .004.
A Model 60 from 1975 is measuring a gap of .004 with an endshake of .004.

One thing I discovered when measuring these was my shims are a little bit wider than the space between the top strap and the gas ring, so I was only able to insert the tips of the shims into the gap. Make sure you are not getting a false measurement, make sure you are only measuring the gap.

A difference of .003 from one side of the forcing cone to the other is just about insignificant. A truly square forcing cone would not make any significant difference in velocity.

Likewise narrowing the gap down from .010. That will have no measurable effect on velocity. As I recall, the shims for reducing endshake are .002 thick, so you will only get one in front of the cylinder. Yes, they will eliminate endshake and will maximize the B/C gap, the shims force the cylinder into its furthest back position. There really is no practical fix for tightening the barrel/cylinder gap short of some major work.
 
B/C gap is measured with cylinder fully rearward - ratchet seated firmly against the recoil shield. There is a myth circulating, that the cylinder stays forward, reducing the B/C gap, when the gun is fired - this is only partially true. The cylinder is dragged forward only until the bullet clears the gap. At the instant the bullet clears the cylinder the expanding gasses rush thru the gap and force it to the back, bringing the gap to it's widest dimension.
 
Right. I have learned to look for a better revolver at the counter before I buy it. I believe the .013 and the unsquare cone face is a reflection of present-day S&W quality and craftsmanship. The problem with trying to have it fixed is the expense. It's only a $400 revolver to begin with. I'd be better off looking for one that by chance was built better. But the shortcoming won't by any means prevent this one from being useful.

I had the same problem with my wide automotive feeler gauges. I bought a set of tapered ones for a few bucks so I can revolver shop with them. Besides measuring the gap, I've learned to check the end shake, lockup, alignment, look for timing issues, and if it has an exposed hammer to check for rounded sears.
 
Mizar said:
B/C gap is measured with cylinder fully rearward - ratchet seated firmly against the recoil shield. There is a myth circulating, that the cylinder stays forward, reducing the B/C gap, when the gun is fired - this is only partially true. The cylinder is dragged forward only until the bullet clears the gap. At the instant the bullet clears the cylinder the expanding gasses rush thru the gap and force it to the back, bringing the gap to it's widest dimension.
Click to expand...

I beg to differ. If there is endshake present, barrel/cylinder gap is measured with the barrel pushed all the way forward.
 
Kuhnhausen is talking about the only practical way for one to measure endhsake (cylinder endplay) and barrel/cylinder clearance. The min dimension is of importance for us not only for measuring endshake, but to make sure that the cylinder will not bind against the barrel. The maximum dimension on the other hand is what we call "the B/C gap". He might be hard to understand, or misleading sometimes as it is expected for one to have some kind of basis (gunsmith, or armorer) and use his books as aid, or repair manual and not like an introduction to gunsmithing.
For those that are not still convinced take a close look at the following video. Start from the 0:30 min mark, preferably with 0.25 times the normal speed. Look very closely - it will answer many questions, especially why the B/C gap is measured with the cylinder pressed to the back. And why, when correcting excessive endshake, we are not opening the gap - it's already opened. What you see on the sides of that revolver when at rest - the smaller gap, it's only cosmetics, something not relevant. You will not get any velocity loss by shimming the yoke - the revolver will perform just like before the intervention.

 
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Isn't the b/c gap measured with case's in the chambers and a feeler gauge with the proper headspace between a case and the recoil shield?
What ever is left at the b/c is the gap as measured.
 
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