Life limited parts

[RL17]

I recently had a bolt shear off two lugs on my AR 15. The Smith hard chrome bolt gave out after a steady diet of 77mk's after about 5000 rounds. I have not sent the bolt back yet, but I was wondering if anyone has an idea of the life limit of the bolt and other high stress parts.

   The load I am using is a little on the warm side, but not anything out of the ordinary for long bullets.

[kurtm]

Hey Pat:

The hard chroming process creates stress rizers in most chrome molly steel. This happens on a molecular level as you get a certain chrome migration into the steel causing an uneven flash hardening causing stress plains that eventually will crack under impact loading, especially in SMALL PARTS with close to 90 degree shoulders I.E. bolt lugs and CAM PINS I don't know how many of these I've seen broken over the years. To a more direct answer to your querry, I have a colt bolt (non chrome) that has lasted for 3 barrles, maybe 25,000 round ( I don't count 223 rounds). I really recomend NON chrome bolt and carrier parts but they don't look as cool! another way to go if you just must have plating would be Nickle, or TiN. Hope this helps and SEE YA IN MESA!!                            KURT

[RL17]

Thanks Kurt. We are looking forward to Mesa and the RM 3 gun.

[ErikW]

That's not good news; I have hard-chromed bolt and carrier parts, but not nearly as many rounds through them. I thought hard chrome hydrogen embrittlement was a thing of the past.

If the cam pin shears will the gun blow up?

[RL17]

I don't think the pin will shear. but I did have a bolt break last year. It broke in the cam pin hole. It extracted the round, then the bolt fell down but managed to strip a round and punch the primer with a lug. It blew the mag out of the gun and the exploded brass was formed around the barrel extension. It wasn't a hard chrome bolt, and didn't have many rounds on it. It was an aftermarket no name part.

 I think Kurt nailed it, I havn't had any Colt bolts fail.

[kurtm]

Eric:

Unfortunatlly Flash hardening is alive and well. The good news is that cam pins don't seem to "shear" they break around the firing pin hole or around the top flat crown. ALL the ones I've seen break WERE CHROME!!! ( some 7 or 8 in the last 5 years) Thank goodness that enough of the pin stays in the cam slot to not let the bolt fire in the non locked possision ( at least in the ones I've seen so far). It does tie the gun up nicely and sometimes requiers taking the barrel off or other drastic measures. I have seen bolts go like Pat mentioned, cracked at the cam pin hole, even a Colt, but this seems more like a "fit problem" between the carrier, bolt and extention on rebarrle jobs. If your bolt is really smacking the back of your barrle INSIDE the extention (not just some wear marks where it touches), you are probably set up for this kind of failure at some point in time. Don't confuse this with HEAD SPACE as that is the distance from the bolt face to the shoulder  INSIDE the chamber.  Some times we have to chalk these failures up to bad parts as Pat mentioned so be careful of what you buy.        KURT