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Luke I was going to post the same thing until I read the rest of the post. I *think* what he's trying to ask is, if the HPT isn't done on a batch of 100 bolts, at what round count could the 3 bolts that should fail be expected to fail and how.
Ahh... Yes, I see that now
On one hand I look at HP/MPI as a procedure that finds the parts that are flawed from the get go. But, that HP is the key. If one never suffered an overpressure condition in their rifle I expect that a number of that 3% would likely serve through their normal expected service life without failure.
Then, you have to figure that a certain number of that 3% would "fail" a MPI inspection at some point but not in a way that was user detectable or catastrophic, and may still serve out their expected service life or at least not be drastically reduced.
Then, some part of that 3% would fail early, detectably, even catastrophically. Those are the ones I don’t want.
Yes, it would be interesting to know the details of the 3%. How many are catastrophic failures? How many are detectable after the HP without the MPI? Stuff like that. Even more interesting and I suppose more to the point of what TehLlama was getting at would be to take a significant number that "failed" HP/MPI testing, but only in a way that was detectable with MPI, stick them in guns and see when they failed in an obvious/detectable manner.
IMHO, the thing that kills bolts the most is poor heat treating. This failure usually shows itself at 3K and under and the break always happens at the cam pin hole.
BM currently holds the record for the most broken bolts in this area.
As your gun is shot, small cracks form naturally. The bolt continues to work just fine for a long period of time AFTER that the cracks have formed.
Someone will be along shortly to say; "My XTZ bolt is heat treated and so it is GTG and don't need to stinking HPT/MPI.
The bolts we see break are usually from companies that don't follow the TDP in any way shape or form. This is why buying from companies that acknowledge the TDP/Govt Std and follow it are always going to have a higher quality level than ones that do not.
C4
I'm still waiting for a company to say screw HPT/MPI we're going to x-ray our products because the HPT reduces bolt life by x percent.
It would not be hard to check the heat treat in a separate step to prove that the operation was done correctly. If a company said they follow the TDP except skip the HPT and replace it by checking the hardness on every bolt, that could be viable. Unfortunately, many of the same companies that don't follow the TDP would likely not have a rigorous enough testing method, leading to the same BM scenario.
There is a large percentage of people who don't care if there is a better way, they want the TDP. It's proven. Until the better way is incorporated into the TDP, it will likely be seen as less desireable by many. Specifications in any large organization are slow to change, and usually it takes overwhelming evidence. The pace may be slow, but they won't get caught up in a fad that looks great at first and then creates problems five years down the road.
Even if there is something better, Colt is making a lot of M4s every day. If there was a rash of bolts or barrels failing at low round counts, it would be known.
I've seen a shit ton of rounds go through Colt M4s. I've worn the barrel out of one in about 3 weeks. Yet I had never heard of a Colt bolt breaking. I never new broken bolts were a problem until I had to carry a BM.
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