barrel-cylinder gap affects accuracy (and other things, too)

S

stevieboy

New member
Back in January I purchased a used Smith & Wesson 625JM, a gun that I'd lusted after for a while. The deal seemed to be too good to be true, the asking price was a couple hundred dollars less than comparable guns, but the gun looked good and I wasn't about to hesitate.

Until now, it has been a big disappointment. It just didn't have the accuracy that I'd expected it to have. In fact, it kind of sucked in the accuracy department. On a good day it would give me 3" groups at 10 yards, 6 or 7" groups at 25. I attributed that to me and my aging eyes, and I attempted to deal with the problem by putting a fiber optic front sight on the gun. That didn't help much, if at all. The gun had other problems. Large amounts of carbon would build up in the forcing cone after each range session and removal was a pain. And, once the gun heated up, after about 20 rounds, the cylinder would start to bind against the forcing cone, making the trigger pull terrible.

I decided to send the gun off to Smith & Wesson for possible repairs. But, last week, on a whim, I showed it to a local gunsmith who's done some decent repairs for me in the past. He looked at the gun for about 10 seconds and said: "I know what's wrong!" He pointed out that the barrel-cylinder gap was virtually nonexistent, that one could see no daylight at all between the back of the forcing cone and the cylinder face.

He assured me that he could adjust the gap for a nominal cost. I picked the gun up yesterday and took it to the range this afternoon. Instant improvement!! No more binding, and after the session, the forcing cone had no carbon buildup whatsoever. But -- and this is the really cool part -- accuracy improved incredibly. At 10 yards I was easily clustering all of my shots in an inch or so. At 25, I got a couple of 3" groups centered around the bullseye. That's as well as I can shoot with any handgun.

So, the bottom line is that the barrel-cylinder gap can affect a lot of things and accuracy is one of them.
 
M

MrBorland

New member
Yeah, it always fascinates me how things in a DA revolver that aren't obviously related really are. Was the accuracy improvement largely due to an improvement in the DA trigger, or did the inherent accuracy of the gun improve?

And what did the 'smith do to fix the gap? Were there a lot of rounds through the gun when you bought it? Mechanically, a cylinder gap would open with use, yet the problem was too little? Did it just need a good cleaning? Or did it come from the factory with a tight gap?
 
S

stevieboy

New member
Evidently it came from the factory with a tight gap. I think that the smith backed out the barrel just a bit in order to create an acceptable gap. Before he did that the gun shot to the left (among other problems), now it shoots dead center. I actually had to recenter my rear sight.

However, the improvement in accuracy was more than just that the gun is now centered. There was a significant spread in my groups that drove me nuts. As I said in my original post, I'm a fair shot and I can generally get any gun that is performing to spec to give me a nice tight group in the 10 ring at 10 yards. I couldn't do that with this gun. I'd get a couple in the 10, a couple just out of the 10, and a couple that would be 2 or three inches away from my point of aim. I wondered if I'd developed a flinch, I switched from double action to single action, I even tried resting the gun, all to no avail.

This wasn't due to the trigger, although the binding and roughness were a distraction. Shooting single action produced no better results for me than my customary double action shooting did.

All of these problems have vanished. The gun is now a tack driver, shooting exactly to point of aim. At 10 yards I was easily able to put 12 rounds in the 10 ring. At 25, my group opened up, but then, and as I've said, I'm no bullseye shooter. I was still able to put 3 of 6 in the 10 and the other three were 9s and 8s. I'll take that any time, but with a 4" gun in double action, I'm really pleased. Before the gap was adjusted I'd sometimes shoot groups at 25 yards where not every shot caught paper.

I have no idea why changing the gap to within spec. would affect accuracy. The smith thought that it might have to do with the angle at which some of the rounds were entering the forcing cone. He speculated that some might have been entering at an imperceptibly slight angle caused by the contact between the cone and the cylinder face and that, in turn, caused the rounds to travel unevenly down the barrel. I'm not really buying that, but, hey, it's more of a theory than anything I can give. I'm also not sure why a tight gap would cause heavy carbon/lead buildup in the forcing cone area but, it certainly did. Perhaps all of the crap that was being ejected directly into the forcing cone was affecting bullet trajectory, I dunno.
 
V

vladan

New member
If the gap was too tight, the carbon deposit after few shots can cause cylinder bind and cause it to be slightly out of time causing inconsistent groups.
Opening the gap allows the cylinder move freely to full lock-up.
 
E

Eagle0711

New member
It likely had excessive head space. That is the increasing gap between the recoil shield and empty cases. The cylinder works forward and rubs on the forcing cone.

The repair can be accomplished in a number of methods. The easiest is a thin washer from Ron Power that is placed in the cylinder at the rear where the ejector rod fits in moving the cylinder back.

Happy to hear of your improvement. Really who cares as long as success was achived?
 
D

Daryl

New member
I have a Ruger SA with essentially the same "problem", except it's not a problem at all. Shoots very accurately, and I've not had any problems with carbon build-up.

I suppose it also depends on the firearm.

Daryl
 
S

stevieboy

New member
I'm thinking that Vladen nailed it. But, whatever,the problem is now a thing of the past.

I'd say to any of you who purchase a revolver, visually inspect the barrel-cylinder gap before purchasing. And, if any of you have one that is acting bizarrely, as was mine, don't overlook that gap as a potential source of problems.
 
J

James K

Member In Memoriam
I doubt the inaccuracy was the result of the gun being out of time. More likely is was the result of carbon or bullet material building up in the forcing cone and skewing the bullet as it entered the barrel.

The gunsmith surely didn't unscrew the barrel; he used a champfering tool to ream the cone and true up the breech end of the barrel. Sounds like he knew what he was doing and had the right tools to do it.

Jim
 
M

MrBorland

New member
Sounds like your gun's gap was tight from the factory, but something that can close the gap with use is the crane screw/plunger assembly. IMHO, it's one of the weak links in S&Ws design - it's the only thing holding the crane & cylinder from sliding forward off the gun - and it can bend with use, pushing the cylinder forward, and closing the gap. When bad enough, it becomes tough to close the cylinder, and when really bad, the whole crane & cylinder can fall off the gun during a reload.
 
You must log in or register to reply here.
Top