Empirical Evidence for AR Supremacy ???

[Spiff]

Ahhhhhh a whole different can of Mil Spec
As for Shot Peening...well lets just say I agree about the crack propagating thing (only due to the crack running into a rounded stress reliever, the shot impact area), but not the "compressive" part. It is a percussive residual, it can only be compression if there is containment...and on this semi-ductile surface we are talking about metal is neither added or subtracted, just moved around so there can't be containment. we are talking about a process that AT BEST only "disrupts" to a depth of .00005, as we are talking about a air hardening heat treated steel. Now this works great for "surface cracks", but does nothing for stress risers created by sharp machining angles and carbon/chromium inclusions in poorly amalgamated material (a "bad pour" in the metallurgy business) so it only deals with surface cracks ( yes they can grow ) and the MPI can't tell you about inclusions...It ain't X Ray.. so we are talking about a superficial treatment and a test that while making one feel good, don't really tell you about your bolt. (These are the exact reasons Reed and John question this process)

I'll admit that I don't have any experience with shot peening in industry, so I'll have to take the easy way out and drop some quotes from a couple of my textbooks...

The most common and versatile of the cold-working treatments is shot peening. It is...blah blah blah...since the area is resisted by subsurface material, the skin is placed in residual compression. The thickness of the compressive layer is usually less than a millimeter. The highest compressive stresses occur slightly below the surface and are commonly of the order of half the yield strength.

and...

....they [shot] make overlapping indentations on the surface, causing plastic deformation of the surface to depths of up to 1.25 mm. Because plastic deformation is not uniform throughout a part's thickness, the process imparts compressive residual stresses on the surface, thus improving the fatigue life of the component.

Since a bolt is hardened steel, I would expect the depth of compression to be quite a bit smaller, like you said.

The only limit to residual stresses that I'm aware of is that they have to sum to zero, otherwise the part'll be flying around and causing mayhem. As long as you have all compressive stresses in the radial direction, they sum to zero, and you're good.

I will agree with you about MPI though...it probably is more of a feel good test than anything. And certainly a rough bolt or one with inclusions is a definite no bueno.

Now since we are talking about failure, metallurgy, and stress risers of which I have a bit of passing knowledge, and since you seem to have a good grasp on this subject....where do bolts usually fail?, the least common is to shear a locking lug, although it does happen , usually on full auto guns that see lots of mag dumps.. Where is it?

If a bolt's gonna let go, it'll usually do it at the cam pin hole. There, obviously, you've got the least amount of material, highest stresses, and a big freakin' stress riser.

For extra credit, What Bolt part fails the most out of all of them...not to include firing pins being cooked by pierced primers? and consumables like gas rings and extractors.

Well, since you've taken most of my choices away , and since firing pin retainers usually stay together (as long as you have a shrouded carrier and you don't put one of those dumb solid retainers in), I'm gonna have to say it's the bolt that goes most often, with the cam pin as second.

 

[KurtM]

"Shot size and material, part material and heat treat condition all play a role."

Absolutely!! and we are talking a pre-heat treated, Air hardened 8620 material shot peened with a # 400 shot @ 200 PSI "Mil Spec" if you were wondering...or about a .00005 depth penetration for the perameters listed ( wonder of wonders...I work in the oil/gas field also) I don't agree... It is NOT a compressive layer as much as a percussive layer....semantics I know, but since there is no containment, and the displaced metal is NOT compressed rather it is "free standing" the "round nature of the shot impact has far more to do with stopping "stress propagation" than "compression". We are not talking a "fairly soft" engine part here such as a vane or rod. we are talking a RC of 38- 42...BEFORE shot peening. I can't think of anything out here in the oil patch, outside of pump seats, that hard and in the Aircraft indutry very few thing are "that hard"....but I digress.

You are absolutely right in the fact that it can extend parts life... but here in Bolt Land ( air hardened, heat treated 8620 material, hardened BEFORE shot peening) I feel we are talking a "preoccupation with inconsiquential increments"....meaning that if the bolt hasn't failed in the first 500 rounds or so.....it probably WON'T!!!

NOTE I stated "I don't buy the compression thing"!!!!!

Other Note.... this is a great debate, and I am NOT saying anyone is wrong.....This is the difference between Reed, John, and Mil Spec! It is all good! I am from the Reed, J.P., camp your milage may varry!

So...after all that....Where do bolts fail? I can tell you why and where, but if this is "such a problem" all you guys should be able to tell me at least WHERE!!

 

[KurtM]

Spiff....... you are absolutely RIGHT...Cam Pin hole....except which side, and why?

Part most likely to fail....CAM PIN! Damn amigo you are good!!! And the great part.....since only TWO companies make them and theuy are ALL "Mil Spec" ....welll.... you get it!

 

[KurtM]

Oh hell Spiff has got it...Left side of Cam Pin Hole... it is the side that is in tension on extraction...with tons of stress risers, and even worse with a "carbine gas system", and no amount of shot peening/ magnifluxing will change that! KurtM

 

[Stephen Cue]

Don't let that go to your head, your still a jackhole.

GET OUTTA MY THREAD PETERCHEESE!!!!

 

[Bobrob]

Feels like there should be a grade issued at the end of this thread, it is definitely good education.

Best regards

 

[CAR-AR-M16]

Cam pin didn't break on this one. Bolt carrier and upper took the brunt of this one.

Extractor didn't hold up well either.

 

[Shadowrider]

Cam pin didn't break on this one. Bolt carrier and upper took the brunt of this one.


Extractor didn't hold up well either.

No way that's a Colt upper. They never do that. Ever, never, ever!

 

[CAR-AR-M16]

No way that's a Colt upper. They never do that. Ever, never, ever!

And it did it with factory ammo too (Federal XM193) or at least that is the offical story. http://www.marlinowners.com/forums/index.php/topic,54654.0.html

 

[KurtM]

Niiiiiiice! Saw one in the late 90 like this with S&B ammo, except the upper didn't split like that one. It also blew out the sides of the mag well. We suspected a slight out of bnattery slam fire....but who knows...GREAT PICTURE...thanks!