One of my j-frames drops the hammer simultaneously with the cylinder locking into place when the trigger is pulled (DA) at average speed. The typical way of checking timing didn't make me feel confident that the bullet was lining up with the barrel before being fired out of it.

I thought that a good way to ensure that the cylinder/chamber was aligning properly with the barrel at firing was to use some snap caps. The snap caps I use are the el cheapo plastic ones that have a flat base, rather than a simulated primer in the center.

The thing about these caps is that they leave a permanent indentation when the firing pin strikes them, AND they have a mold/seam line that bisects (pretty accurately) the circle of the base (where the primer would be). This means the seam line goes through the center of the circle, where the firing pin SHOULD strike if the cylinder and barrel are correctly aligned.

I put the caps into the cylinder, lining up the seam of the cap by sight with the center of the cylinder (ejector rod). This ensures that the seam will be vertical when that specific chamber is lined up with the barrel.

I shoot through the cylinder of snap caps, then open the cylinder to see where the firing pin indentations fell. If the indentation is to the LEFT of the seam on the cap, then the firing pin is hitting the primer prematurely, BEFORE the bullet is completely aligned with the barrel.

If the indentation is on the seam line, then the pin is hitting the primer at the correct time.


Any thoughts on this method? Only problem I can see is that it costs 50 cents per cap, or $2.50 for a j-frame. You wouldn't do it all the time, only on guns you had a question about.

Maybe the ultimate test question is.....does it spit?

I have a model 640 that does not lock-up (cylinder stop does not hit the notch) until the very end of the hammer travel, or what I perceive to be the end as I can't actually see the hammer. To me, the stop should lock in place at "shortly before" full hammer rearward travel. BUT....the gun shoots fine and doesn't spit no matter how fast I shoot it.

Snubbiefan, no my gun doesn't spit or show any bad signs that I can tell. So I guess my test was mostly academic, in this case.

I was mostly concerned about DA rather than SA with this gun, too.

My firing pin was hitting on the seam line (in the middle) of the snap caps on all chambers. Actually, with slight inclination towards the RIGHT, not left of the line. Left of the line would be premature firing.

I think the faster you pull the trigger, the more likely you are to avoid premature firing/timing, as the momentum of the cylinder when it's moving fast gets it into position quicker.

I think there's a fair bit of "margin of error" built in to the standard test for timing, as my revolver would've been right on the edge of passing that test. My snap cap procedure seemed to show everything is working as it should, though.

I am an engineer by training and trade (now retired), but I remember the early days of slow-mo photography and how amazing it was to see how things will move and bounce in normal operation. There is a video out there someplace of a Model 29 firing. It's amazing to see what all bounces and moves that we could never witness at normal speed. I don't know how many things they eventually did to keep that gun together under fire, but to watch the video....it would almost make me not to ever want to shoot one, especially the early models.

Sounds interesting. I'd like to see that video. Guess it's like the wings on a United jet flexing - disconcerting at first, but maybe better than having them perfectly rigid? Just a thought. Thanks for the reply.

If the gun didn't flex a bit during firing it would probably shatter or crack. That is why gun barrels are rockwell 38-43 instead of 58-60. Harder and more rigid is not always better. As far as the op's method for checking timing.. does the gun spit is there excessive build up in the forcing cone? or misfires? If the answer to these questions is no and the accuracy is good the gun is fine. The center of a circle is the center no matter how the snap cap is aligned, if the firing pin strike is centered they you are good.

My first 642 did that. I could very very slowly pull the trigger and as the cylinder rotated I could see the timing change. On one hole the hammer would drop before the cylinder locked. On two of the holes on either side of the early one the hammer drops at the same time as the cylinder locked, and the two opposite the early one were was fine. On the one that was early, I could stop the hammer just before it dropped and see how much 'off' the cylinder was by trying to manually turn it the rest of the way. As soon as I touched it would click in place, and didn't seem to actually move. It was that close. And it didn't spit so I ignored it.

I returned the gun for another problem and they replaced the frame. Seems to exhibit the same off-center cylinder condition, maybe a micron or so better, and it doesn't spit. I'm good with it.


(I've been in quality control / quality assurance for years, I notice the tiny stuff.)

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Marty 4513TSW 13-1 642 60-10