Diagnosis needed

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RON in PA

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I purchased a S&W model 10-8 with a 3" barrel and round butt 2 weeks ago and have run into a problem. The ex-police revolver while vintage 1981-82 appears to have never been issued based on lack of holster wear and minimal cylinder ring. The problem is unpredictable failure to fire when shot DA, perhaps 15% of the time. This happens with both handloads and factory ammo. I've never shot the gun SA. The hammer nose appears to be fine. Can happen clean or dirty. My take is that the main spring is weak and needs replacement. Any other ideas?

Thanks in advance.
 
First thing to check is to see that the strain screw at the bottom of the front strap is tight. Also, check to see that nobody has shortened the strain screw to make the spring tension lighter. Most likely your problem is one of these two items.

-Bob
 
And if the mainspring and/or strain screw haven't been monkeyed with measure the cylinder/barrel gap with feeler gauges while (1) holding the cylinder back as far as it will go, and (2) while pushing the cylinder forward as far as it will go. Take the first reading and then subtract the second one. The difference would equal end shake, and could cause a problem.

Report back the two readings and well see.
 
Have you inspected the cartridge that was struck, but not fired? If the dimple is light, it might be the mainspring, but if the dimple is way off center, you've got a timing issue. Post back, and let us know.
 
The strain screw is pristine and on the rounds not fired that I inspected there was a centered, light dimple. One thing that's occured to me is the possibility of the cylinder stop not functioning when fired rapidly DA. I don't have feeler gauges of the proper thickness to test end shake. I ordered a new mainspring from Wolfe, I'll see what happens.
 
Swing the cyl. out and unload gun. With cyl. still out and empty, push the cyl. retaining latch back and hold it there. Now pull the trigger and hold the trigger back to observe how far the F.P. nose protrudes from breech face. Hopefully you have another gun to compare your observation with. If end play is ruled out and the above comparison proves out, it's got to be mainspring. S&W 10-8 has F.P. in hammer right, or not? Somebody here had a 642 light strike problem, but that was frame mounted F.P.. As I recall it took a trip to the factory to replace a short F.P..
 
Yesterday, 3/16/06 was the first day of my retirement and also the first opportunity to tryout my 3" model 10-8 with the new mainspring I received from Wolfe. All 125 shots of factory ammo fired DA went boom so I'm quite sure the FTF problem is solved. Thanks for all the suggestions, luckily the simplest solution worked. There is a noticeable difference (heavier) in trigger pull with the new spring installed and I wonder if an out of spec spring got out of the S&W factory. Love those Smith 38s, especially when they work.:)
 
Check main spring screw...... and then who cares, replace all of the springs.
 
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