B/C gap question

[fakahatchee]

Purchased a mod 27 classic in march, cylinder gap was 12 thou on one side and 13 thou on the other side, contacted S&W warranty and they said that it was excessive and to send it back. Got the pistol back in five weeks with a new barrel installed, so they will correct it.

 

[JackM]

Purchased a mod 27 classic in march, cylinder gap was 12 thou on one side and 13 thou on the other side, contacted S&W warranty and they said that it was excessive and to send it back. Got the pistol back in five weeks with a new barrel installed, so they will correct it.

That’s the impression I got. Well see.

 

[chief38]

In general, a larger B/C gap means more loss of velocity at the muzzle. It is also a recipe for more lead spitting. If down the road end shake needs shims to reduce it, the B/C gap will also increase to unacceptable levels.

Like a few have stated, for most of the last century the B/C gap was .002" - .006". I personally believe in those spec's and would not be happy with a B/C gap of more than .006". Unfortunately, S&W raised their tolerance to .012" which to me is just too much and if it were me, I'd personally ask them to correct the B/C gap to at least half of that.

 

[6string]

In an era where so many Kool-Aid drinkers tout the superiority of modern manufacturing, usually spouting something or another that includes “CNC eliminates the need for hand fitting”, you would think maintaining a B/C gap tolerance of .003-.006” would be a simple matter.
If nothing else, it would provide a reference point to prove their boisterous claims.

But, no.

After decades of being told how important B/C gap is to revolver performance, the same gaslighters now say, “No big deal! .012” is good enough”.

OK. Thanks.

Meanwhile a guy with a frame wrench and the skill set to use a hand file can repeatedly set back a barrel with a consistent .002” B/C gap.

 

[steelslaver]

I do believe modern machining and part production is superior. But, it is all for naught when coupled with poor assembly personnel, methods and control and then compounded by a lack of solid quality control.

Large gaps, poor forcing cones, poor function are not the result of the initial machining. They are the result of shoddy, we don't really care that much, meet your quota assembly culture. That mind set doesn't usually start at the bottom and once entrenched it is difficult to turn around.

If the factory gave me all the parts needed to assemble 100 guns of one model, I would bet I could produce over 90 nice ones an a few that would never be right without several replacement pieces. Some of the completed ones would take way more time to assemble than others. Unless your initial parts production is 100% perfect, your going to find minor mismatches that need dealt with.

When I assemble one from were I swap parts around from various gun, I sometimes get lucky, but often have to do some fiddling. The barrel don't time just right, the center pin is a bit long or short, same with rod, the DA sear isn't quite right etc etc. All those take time to take care of.

 

[raljr1]

I agree that modern machinery allows for tighter tolerances. But think about how barrel to cylinder gap is generated. You have a design that specifies the length of the barrel, the length of the threads, etc. But getting that thread to start at the precise location to get the exact gap is not easy. If you've ever changed a barrel, you know that no two are alike as to clocking or B/C gap. So it you have too little gap, you cut material from the stick thru, if you have too much, you cut from the shoulder on the barrel. if it clocks correctly but has too much gap, now you are cutting a full turn. Easy to see why the manufacturer would not bother for a few thousands that does not matter to the vast majority of gun owners. The only gun I own for which I know the gap is one I replaced the barrel on. And it took several tries to get it right, including filing the front of the frame. After that exercise I decided that I was a mediocre machinist and a worse gunsmith.

 

[muzzlestuffer2012]

I'm not sure what revolver the op is referring to but if I recall when I was working on my 617 and 629 when I got them power customs had some type of shims to close that gap, I remember Ron said the ideal gap is about 6-7 thou.

 

[steelslaver]

On the 2 piece barrels like the 442 the gap isn't set by the shoulder like on the one piece barrel. It is set by a shoulder on the forward part of the barrel pressing against a ledge in the shroud. You do not have to remove enough for the barrel to make a full turn as the sight and ejector lug are on the shroud which doesn't turn. If you removed .006 from the shroud ledge OR the barrel shoulder. the barrel would turn in .006 further while the shroud remains in its original position.

I have adjusted the gap on numerous 1 piece barrels and installed 2 of the 2 piece barrels. A 2 piece barrel would be easy to adjust once you get them apart. I stuck a piece of square stock in the barrel and then poured it full of low temp solder. I have some that melts at 138f. Then used a normal frame wrench on the frame and use the square stock sticking out the muzzle to turn the barrel. Once you correct the gap a heat gun on low will melt the solder and a couple of passes with a brass brush and your good to go. I was making a 38 +p magnum J frame into a 32 mag

On my 500 I cut the shroud down, threaded the barrel and made a nut to fit. Just set the gap and tighten the nut and your good to go. Very similar to a Dan Wesson

There is no great way to close the B/C gap with shims. You must either move the cylinder forward or the barrel back. Just moving the cylinder forward will create more headspace and endshake. You could trim the yoke shaft then put a shim on the ejector shaft so it was between the back side of the star and the cylinder, which would move the cylinder forward and eliminate the added endshake from shorting the tube, and it would hold the ejector back letting the rim headspace correctly on ejector arms, but not on cylinder face itself. You would also have the shim riding in and out on the ejector as you used it.
It would work, but not the best option which is always going to be make sure the yoke fits right, then set the headspace, adjust the endshake, then adjust the barrel to the face of the cylinder.

 

[series guy]

Folks are taking issue with one arbitrary number being replaced with another arbitrary number. There really isn’t any science involved here that can spit out a perfect number. In my world the photo depicts the result of a .006 barrel/cylinder gap. It renders a D/A revolver inoperable in short order. Also the target has no way of knowing the speed of the bullet so that is mostly a moot point. I have opened up more than a few barrel/cylinder gaps. Never had to tighten one up and never had a problem with lead spitting. Also I don’t bother measuring the gap unless the gun gives me problems.

 

[steelslaver]

There is no way that gun had an actual .006 gap unless it had problems like huge endshake, the yoke was way messed up or it had a misalignment or forging cone issue and lead was left to built up for hundreds of rounds.

I have sent a lot of cast lead bullets down range from 45s 44s, 357s, 32s and 22s with a lot of round between cleaning and have never ever had that from a gun with an actual .006 gap.

There is a very simple way to tell the speed of bullets and that is to fire them over a chronograph, which are now relatively inexpensive and easy to use.

I do doubt that an .012 gap loses 30fps over a .006 gap and I agree that the velocity loss velocity won't be what makes or breaks the result of the shot

I will also say that I believe a little play in an actual defensive hand gun is probably a good thing. If everything is minimum tolerance any piece of lint or other debris is far more apt to tie up a tight gun than a slightly loose one

 

[muzzlestuffer2012]

Without taking all measurements it's hard to make a call on the issue for sure but I notice everyone (not necessarily here) makes a big deal about losing velocity I could care less about it I'm more concerned with getting blasted with burning powder maybe that's why my Freedom arms revolvers are .003 or less in the end if it's accurate I would leave it .

 

[Boost]

Did you ever check the timing with a range rod? Lead spitting is associated with lack of cylinder/barrel alignment. (ie worn cylinder lock)

I didn’t unfortunately. I did not know much, if anything, about revolvers at the time. When I took it to get diagnosed the gunsmith just told me the gap was at the upper end of what was acceptable at the time. The spitting part really killed it for my wife. I got her a gp100 then but “she’d been hurt before”.