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I recently acquired a S&W 625 .45acp revolver.  I'm sure it was owned by a competitive shooter because the trigger has a stop installed on the rear surface and there is a ball detent in the top of the crane which securely locks the cylinder in place.  The problem is when the trigger is pulled quickly for rapid fire the cylinder often jumps two chambers rather than just one.  It cycles normally in slow fire and single action.  Upon initial examination I determined the hammer block had been removed.  I understand this is a common but by no means safe procedure to speed up the action of the revolver.  I replaced the hammer block but it made no difference to the skipping problem when in rapid fire.  All this testing has been done with a empty pistol.  I'm not about to risk life and limb with a mistimed revolver.  Returning it to S&W is an option but I'm comfortable replacing parts on the gun if I know where to start.

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It's been some time since I was inside a revolver, but it seems to me that it would have to do with the cylinder lock.  It is not rising to engage the notch in the cylinder in time.  That may be a spring problem, or that the lock itself was modified (perhaps shortened) such that it rises late in fast cylinder rotation.

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Posted (edited)

Most likely there are burrs on the stop notches on the cylinder. take a look at the side of the notch that is relived, right at the end of the relief where the notch is it is likely you will find a small raised edge where the metal has been peened up and can cause the cylinder stop to jump the notch. you can carefully remove this lip back to a square corner (not round) and it will likely improve the problem. also look at the cylinder stop make sure it is moving freely and not gummed up and moving slowly for any reason, also ensure its smooth and square.

 

The hammer block is really only there as a drop safety, as in dropped with the hammer cocked, so it a common modification on competition guns because they are almost never manually cocked. (mine are never manually cocked)

Edited by MikeBurgess
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If you give us your general location, maybe someone local can help you in person. In the meantime, here are a few things to check out.

The main reason to remove the hammer block is to make the trigger return easier with a light rebound spring. That is a bad idea, as you correctly surmised. The way to make it work with the hammer block in is to smooth up the long sides of the triangle hole in the bottom so the pin on the side of the rebound slide can move in the hole smoothly. Also, flat file the sides and make sure it slides freely in the slot of the sideplate. File a generous chamfer on the front and back sides of the top of the flag so it can return upward without catching on the frame or the hammer.

 

When a revolver is used a lot, especially at higher RPM, the inertia of the cylinder turning, then coming to a sudden stop, creates burrs on both sides of the cylinder stop notches. It's likely that the burr on the ramp side is causing the stop to bounce down out of the notch when rotated quickly. The fix for this is to peen the metal on the cylinder back where it came from. On the ramp you can use a 5/16" round steel bar on it's side (the size and shape of the ramp) and tap the bar with a hammer to iron it out. On the other side of the notch, you can use a flat ended punch (maybe the same 5/16" bar) using the end of the punch to move the material back where it came from. If this does not fix the problem, let us know, with more information and we will try something else.

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I've checked the cylinder lock notches for burrs and deformed metal and I have been unable to perceive any.  The cylinder lock itself is shiny on one side but does appear to have worn away any metal.  I was advised to check the cylinder lock spring and while it is in place and returns the lock when depressed I have no way to determine whether the spring is correct or has the required tension.  I'm in SE Texas between Huntsville and Livingston on the north end of Lake Livingston.

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I always figured they gave up some spring load on the cylinder stop when they dropped the 4th screw and required the stop spring to be crammed in place from the side.

Wolff makes regular and extra strength stop springs.

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