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Member
I can't say for certain.Originally Posted by Submariner
That heat treating info would have to come from those manufacturers.
Shot-peening is a stress-relieving process, and I wouldn't think that would have anything to do with the environmentally-caused surface micro-pitting.
Industry Professional
The heat treatment of the bolt is not complicated just bloody awkward. Every manufacturer including both Colt and FN contract this out to dedicated heat treat houses who are able to keep their furnaces in better calibration as they are used continuously.
Uneven case hardening (the bolt should have 0.010" to 0.014" effective case before final grind) will not contribute to the fatigue failure of the part. Case hardening is a diffussion process so it will naturally vary to some extent even before machining. On the plus side the case will sit under slight compression so will to some extent offset tensile loads. With this in mind, considering the further manufacturing processes, and remembering that case hardened layers are brittle, the process of shot peening must bear scrutiny. If it is to provide compressional loading as intended then the detrimental effect on tolerances and the microcracks that form in the case vastly outweigh any positive benifit. Conversley a very light peen to refine the surface might be of benifit but the outcome is so slight as to be academic.
The microscopic pits observed in the study by Yu, Kohl, Crapanzano, et al are somewhat bigger than microscopic and are definitely nucleation areas for fatigue cracks. I think the emphasis on these was somewhat brushed aside to show the modelling (not uncommon or unreasonable in the presentation of a scientific paper). In terms of bolt life under a particular loading, these may be the dominant factor and very little was done to explore how or when they were formed. Remember that the steel involved bears both chromium and nickel so it is not prone to corrossion. Without more detailed information I would be loath to air my hypothesis with regard to this but I am continuing work along these lines.
Barrels that are ported correctly for timing usually exhibit excellent resitance to changes in port size. From recent work on a 10.5" barrel in the 6.5 we established correct function at 0.094" dia. Funtion could be maintained down to 0.088" but the gun then ran consistently as the port was opened out to 0.109" Cycle speed became excessive at 0.116" but function remained.
Bill Alexander
Banned
JUST WONDERING what ammo was used?
The empty Brass thats in pic with red primmer sealant Q3131 maybe
or adcom M855
Member
also some manufactures bolts are also casted which was shown in one of the pictures on the first page, Also, a few manufuctures do not proof there bolts and check them for cracks.
Senior Member
Absolutely.
Those lovely angle-drilled MRP gas ports are done that way to reduce the effects of erosion over the usable life of the barrel.
Without the sharp edge of a typical 90 degree gas port, it's not going to START eroding nearly as quick.
Randall Rausch
AR15 Barrel Guru
Senior Member
Very interesting. I know someone (Federal LEO) that put 20K through a HK416 14.5", 5K through a Colt M4 14.5"and 5K through a LMT 14.5" (non MRP). He then had the barrels cut longways in half to see the differences of the three.
The LMT had the largest amount of gas port erosion.
Chief Armorer for Elite Shooting Sports in Manassas VA
Chief Armorer for Corp Arms (FFL 07-08/SOT 02)
Industry Professional
The bolt failures at the cam pin hole are the result of the sudden rearward whack delivered to the bolt by the cam pin when the front of the carrier's cam track whacks the cam pin. The simple inertia of the front part of the bolt causes stress in this area, which is, basically just too thin. Any little variance in heat treat or material really gets a chance to make itself known quickly since the bolt in this area is under built and not overbuilt. Not a criticism of the original design, it's just one of those things, an unintended consequence of the many redesigns and mods done to this system over the years. Carrier runs faster in the shorter gas-tubed models, so it's whacking the bolt harder.
I've suggested to a few manufacturers that a second bearing band be put on the bolt, centered on the cam pin hole. Material thickness would be increased about 11%....... that's worth having, I think. It's like, "the quickest way to more horsepower is more cc's". The quickest way to more strength here is not a new, super-exotic material or process-- just add metal. The bearing band would make it not-mil-spec, sure, and it might even bring it's own unintended consequences.... but it seems like it ought to be worth trying.
Member
Ned - thanks for posting that info. I actually witnessed a bolt failure just like you described at a competition this weekend. I don't know much about the guys rifle, but it was an RRA rifle with only about 2-3K rounds through it over the year -year and a half he's owned it.
Industry Professional
There is so much deep reading on this thread-- going over it again I find that what I just said is mostly a reiteration of what Bill Alexander said months ago:
"The last area to consider in the failure of the bolt is cracking at the cam pin hole. Beyond the metallurgy of this thin section higher carrier group acceleration will impose increased load at this area but the preparation of the edges particularly where the swage marks are applied must be considered. The fit between the cam pin and the bolt is also a major source of loading for this area. Typically cam pins run to the small side and the cam pin hole in the bolt is to the large end to allow easy assembly. This also imposes bending loads on the bolt and a much tighter fit while not as easy to use in the field will extend the life of both the bolt and cam path (assuming the basic physical properties are present, you cannot compensate for crap)."
Banned
Discussions like this is why I go nowhere else to learn about ARs.
Outstanding.
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