S&W940 ammo sensitive?

K

Ken Dunham

Inactive
Recently I got a used S&W940 which after only a few rounds has had the cylinder bind twice (and need to be leather-hammered open). The binding appears to be at the point where the round just fired (the cylinder will not advance to the next round) expands into all the space between the frame and rear of the cylinder. Visually when fresh moonclips are loaded, there appears to be adequate clearance. It is only after firing that the cylinder locks up. And in dryfire the cylinder appears to be properly aligned.

My instincts are to blame the brandX moonclips as not being stiff enough. But the gunsmith suggested that certain brands of ammo might have a primer raised just enough to bind the cylinder? More experiments with ammo/moonclip combos are clearly in order, but can an experienced 940 practitioner enlighten a very frustrated revolver shooter?

FWIW my S&W625 has had the cylinder seize up because of bad full moonclips before I started using quality moonclips from Ranch Products. It has never had a problem because of ammo.
 
I have not had a problem with binding like that. Mine is that the gun gets tight after it heats up from a few cylinders. I do have what I think is a combination tweaked crane and poorly cut rachet notches on the extractor star, that the gun has to go back to S&W for to look at. The problem I had was extraction when using CorBon ammo. The empties stick to the cylinder walls. I am going to have the chambers polished.
 
I have been shooting a 940 for a couple of years now. I have not had any problems except with the snappy recoil with hot ammo.
Are you using reloaded ammo? If so, what primers are being used? Federals are softer and have a tendency to flow under higher pressures in "small pistol" size. Also have you checked to make sure there is no unburned powder under the extractor star? That will bind up a wheel gun right quick.
Anyway, I hope you get it figured out. I really like mine. In fact I shot it in a match Wednesday evening as a second gun.
Neil Casper
 
I've had the same problem. Unfortunately I bought mine used and couldn't get a free fix. This is a fairly frequent complaint about the 940. The cylinder clearance is too tight and needs adjustment. The primer bulges and the cylinder locks up. S&W can fix that. I traded mine. Others have found ammo that works, but I don't know which type.

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Mark
NRA member
 
In all probability there is nothing wrong with your 940. The problem is that of using a >tapered< case in a revolver. This same problem was encountered by the Israelis who longed for a 9 m/m revolver to provide reasonable "stopping" power and ammo interchangeability. Factors that encourage the fired case to recoil towards the backplate and thus bind the gun include: slick chambers - either from excessive lube, or polishing. Nickel cases (less friction). A powder charge/burning rate ratio which is not compatible with the type and weight of projectile. Changing primers and/or cartridge retainers does not provide a solition. S&W attempted to resolve the problem by using a radical phosphor-bronze finger design to retain the cartridges, but the manufacturing costs and additional other problems resulted in project termination. In theory, a 9 m/m revolver is great. But don't stake your life on one..... at least not if more than one or two shots are required. You might experiment with dry chambers, brass cases, and assorted powders and bullets to minimize the problem. If you find a working combination, best to stick with it and leave well enough alone. It's truly a shame, but that's reality.
 
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