Mil-C-70599A is the MILITARY SPECIFICATION for the CARBINE, 5.56MM: M4. This document is readily available online dated 20 January 1994. It is exactly what the name implies. Relevant sections for this discussion are:
3.4.4 High Pressure resistance. Each barrel assembly and
bolt shall withstand the firing of one Government standard M197,
5.56mm high pressure test cartridge conforming to MIL-C-46936.
After proof firing, parts shall be free of cracks, seams and
other injurious defects as evidenced by visual and magnetic
particle inspection.4.7.4. Barrel inspection. The barrel assembly shall be
magnetic particle inspected in accordance with MIL-STD-1949
utilizing a current of 400 to 500 amperes for circular continuous
magnetization. The barrel assembly shall be examined for
evidence of cracks, seams and other injurious defects.4.7.4.3 Bolt inspection. The bolt shall be magnetic
particle inspected in accordance with MIL-STD-1949 utilizing
standard five turn magnetizing coil with a current of 200 to 300
amperes. Both circular and longitudinal continuous magnetization
with wet fluorescent solution shall be used. The bolts shall be
examined for evidence of cracks, seams and other injurious
defects.The document makes reference to the following other military documents relative to the above4.7.4 High-pressure resistance test. This test shall be
performed during individual carbine testing in accordance with
TABLE III, using a fixture for holding the bolt and barrel
assembly per drawing 11837944.
36
MIL-C-70599A(AR)
4.7.4.1 Test cartridge. One (1) high-pressure test
cartridge (see 3.4.4) shall be fired in each bolt and barrel
assembly. Unless otherwise specified, the bolt and barrel
assembly shall be tested concurrently. After proof firing
cartridge cases shall be examined for bulges, splits, rings and
other defects caused by defective chambers of the barrel
assembly.
MIL-C-46936 Cartridge, 5.56mm, Test, High Pressure, M197
MIL-STD-1949 Inspection, Magnetic Particle (this is a 27 page document, also available online)
INSPECTION EQUIPMENT DRAWINGS 11837944 - Barrel and Bolt Proof Testing Fixture
There seems to be a lot of discussion of the minutia of the above in various locations recently. In order for one to have an intelligent discussion on the topics, knowledge and understanding of the above documents is necessary. Additionally, it is very easy for a manufacturer to say "we HPT" and/or "we MPI", when in fact what we as consumers should be looking for is "we HPT in accordance with MIL-C-46936" and "we MPI in accordance with MIL-STD-1949".
This by no means excludes other means or methods of quality control. However, just like a builder must demonstrate how their way is better than the building code if it does not meet it, manufacturers should be expected to demonstrate how their method of quality assurance exceeds the requirements of the relevant documents above.


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