What is the benefit of a full auto bolt?

Tommygunn · Jan 23, 2019

AlexanderA said:
What is labeled in the picture as the "SP1" bolt carrier is actually the "post-moratorium" beveled, or unshrouded, bolt carrier. I have two SP1's, that I bought new in the "pre-moratorium" period (circa 1968), that both came with shrouded carriers. (The "moratorium" was circa 1970, when Colt temporarily pulled the AR-15 off the market to address the disconnector-removal issue.) The unshrouded carrier is nothing but trouble. In particular, you have to be careful to use a small-collar firing pin, or else the gun may jam even with the disconnector in place.

I bought a Mossberg MMR with a unshrouded carrier ..... replaced the whole assembly with BCM BCG I had handy. Afterwords I made a point to ask if I could open the action up on any AR I wanted to buy.
This happened in just the last year or so....so those SP1 carriers must still be out there .....

kscharlie · Jan 23, 2019

Owen said:
Those aren't different steels, they are different testing methods. HPT is High Pressure Test, MPI is Magnetic Particle Inspection.

HPT means that it fired a proof load without a visible failure, and it still passes headspace

MPI means that is was exposed to a magnetic particle that glows under black light. The part is basically dunked in a fluid under a strong magnetic field, then removed, rinsed off, and examined under a black light. If there is a crack, the crack will glow under the black light.

Poor sentence structure on my part. It was not intended to imply that HPT and MPI were types of steel. It was supposed to convey three distinct things: 1)Type of steel 2) HPT 3) MPI. Thanks for adding the explanation of the terms. I knew what they meant, but am sure there are many who do not. I should have included them.

What is the benefit of a full auto bolt?

Tommygunn

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kscharlie

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