cyphertext
US Veteran
MPI and HPT is recommended, but there are quality bolts and bolt carriers out there that aren't Melonite, Nitrided, or Nickel Boron that function just fine.
Spare Bolt?
- Thread starter enacht
- Start date
MPI and HPT is recommended, but there are quality bolts and bolt carriers out there that aren't Melonite, Nitrided, or Nickel Boron that function just fine.
hdwhit
Member
Why?
What mechanism of action or mode of failure are you positing to explain such failures? Mind you, I'm not disagreeing with you, but I don't understand why you are ascribing the failure to torque or tension of the bolt rather than other mechanisms such as failure or the material to meet specification(s).
BTW, If you have basic measuring tools that are accurate to the nearest thousandth, you may be able to compare bolts to each other. Measure the lug thickness and the drop to the bolt face. If the net numbers are close there should be no dramas. (I use a 0-1" mic. and depth mic. but you can use calipers if you have a good feel. Not as reliable, but most reloaders have 'em.)
All I've ever dealt with are MPI'ed "fancy pants" bolts from Colt, Knight's, USGI, etc. and have yet to detect more than 0.002" variation since the 1980's.
Ameridaddy's barrel extention drama (Post #30) or chamber variations are far more common. And it won't matter what bolt you install, it'll still be messed up. (Problems I've yet to have myself but have witnessed it on many occasions.)
All I've ever dealt with are MPI'ed "fancy pants" bolts from Colt, Knight's, USGI, etc. and have yet to detect more than 0.002" variation since the 1980's.
Ameridaddy's barrel extention drama (Post #30) or chamber variations are far more common. And it won't matter what bolt you install, it'll still be messed up. (Problems I've yet to have myself but have witnessed it on many occasions.)
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A. MPI of every bolt is a good thing but some companies only do batches to save money.
B. HPT of a quality bolt will shorten the lifespan of said bolt by up to 50% say the good folks of KAC.
C. Fads not needed on spec parts.
Rule of thumb for carbine gas used to be replace the bolt when you replace the barrel at around 10,000 rounds. Wear point on carbine gas barrels is gas port erosion, a chrome lined barrel would last well beyond 10 K but as gas pressure went up it would put more stress on other parts such as bolts and lead to failures. This is showing up with the new M855A1 ammo which runs at almost proof pressure and is wearing out guns faster, bolts are breaking at around 6,000 rounds on guns using this ammo, but the Army has a solution, replace bolts at 5,000. Back on subject, while there are exceptions a spec bolt on a properly gassed rifle should last well beyond 10 K. But since anything made by man can fail at any time (a few years back Colt released a bad batch of bolts that were losing lugs at 3-4 K) and since they are dirt cheap right now why not stock spares.
The stresses the bolt experiences at the cam pin area are during chambering/locking and unlocking.
During chambering/locking, the cam pin pushes the bolt forward and torques it to locked position. This impetus occurs from the decompression of the recoil spring, via the buffer.
During unlocking, the cam pin torques the bolt to unlocked position, then yanks it back with the recoiling bolt carrier. This impetus occurs with the gas pulse acting on carrier.
I'm not an engineer, but I'd assume the gas pulse accelerates the carrier more violently than the compressed recoil spring. The majority of bolt mass is in front of the cam pin, so rearward acceleration puts more tensile stress on that area of bolt (cam pin port?) than when it's pushing the bolt forward on the chambering/locking cycle.
Hope I explained that clearly. It was clear in my mind...
"I knew exactly what I was thinking about..."
Aside from the lug failure batch, are the 6K-round count failures typically at the cam pin 'port', on the bolts?
