Weird Half Moon Mark on recoil Shield of 66 No Dash

H

Homerboy

Moderator
Gun is in fine shape. Locks up tight and no front to back movement in the cylinder. But it has this mark around the firing pin hole that is a perfect matcg to a shell casing. it's etched into the metal, not dirt. Any ideas? Gun fired fine yesterday with 158 grain .357 and .38's.

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Cylinder slightly misaligned with barrel?

You can get better closeup pics by moving the camera back slightly so that the marks are at a distance where the camera can focus. Then you can magnify the image to make the impacted area larger or simply crop the image and resize.
 
it has this mark around the firing pin hole that is a perfect matcg to a shell casing
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Yep, it is a perfect match for a casing because that is what causes that mark. When you fire a cartridge, the case recoils backwards against the recoil shield as the bullet travels down the bore (that's why it is called the recoil shield). It's that whole Newton's 3rd Law of Motion thing, equal and opposite, you know. The case peens and burnishes the steel on the recoil shield. It's normal, it's entirely safe, it is predictable, and we smiths actually use that wear to tell if a revolver has ever been fired and how much (in relative terms).
 
Um yeah, Scorch, I think he understands that it was caused by the impression of the case. Why is it a half moon and not a full moon? What does Newton's law say about imbalanced impressions/depressions?
 
Newton's Laws don't say anything about ammo loaded out of round and not totaly square to anything. That's for shooters to post about because they don't get it.
 
I just checked my 686 that has a few thousand round through it and it has a round stain but nothing like the one in the picture. have you shot some hot loads in it?
 
Roughedge, I have fired exactly 50 rounds of .38 and 6 158 grain .357. Just got the gun. I'm finding it hard to believe it was shot with super hot loads because the forcing cone is not cracked and the gun is tight. No wiggle front to back or side to side on the cylinder.
 
Firing pin has peened hole in firing pin bushing. Clean it up with a stone. Over the years I've seen this on numerous law enforcement ranges. I wouldn't worry about..but if you do, S&W can replace bushing, which would be knocked out, and a new one swaged in.
 
Firing pin has peened hole in firing pin bushing. Clean it up with a stone. Over the years I've seen this on numerous law enforcement ranges. I wouldn't worry about..but if you do, S&W can replace bushing, which would be knocked out, and a new one swaged in.
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You can see that in the pic?
 
I'll have to do that next time I shoot it. The cases are in the bin with all my other .38 brass. I have no idea which were fired by this particular gun.
 
My uneducated guess would be gas erosion from leakage around a case that isn't expanding completely,
 
**Update** Just got off the phone with a S&W authorized repair center. They believe it's a tool mark. Gun fired and functioned fine. Timing and endshake are all good. The guy even remarked how the gun looks new, which it does, despite being nearly 40 years old. Guess someone fell asleep at the switch the day it was made. No matter. Gun is still sweet. Looking forward to getting it back.
 
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