C = Colt  (B here = Bushmaster, FN = FN Manufacturing, K = SACO, etc.)

M = Magnetic Particle Inspected
P = Proof-load tested


-Troy

You may know it under a common brand name: Magnaflux.

By charging the metal with a magnetic field, you can drop flakes of metal on the barrel, and anywhere there are cracks, the flakes will make a distact pattern.



It is a common, non-destructive way to check for fractures and other steel problems.

-Troy

This is semantic as the MP mark, magnetic particle inspected, or magnetic particle inspected and proofed -- take your choice,  is put on all at one time after both procedures are conducted.  Procedure starts with proof firing but there is no mark for just proof fired  only for the complete procedure of firing and inspection.  

A case could be made either way and the milspec is non-specific.  Logically since proof firing is done before MP inspection the mark should be P MP or PM if it was intended to mean proofed & inspected.  Regardlesss MP is the final inspection mark for a barrel or bolt which has withstood proof firing and a subsequent magnetic particle inspection.

This is from the M4A1 spec:

3.4.4 Hiqh 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.

4.7.4.4 Proof markings.  Proof marks and magnetic particle inspection marks shall be applied on barrel assemblies and bolts that have passed this test.

-- Chuck

From Page 37 of the MIL-R-63997B(AR) the M16A2 "milspec"

4.7.4.4 Proof Markings. Proof marks and magnetic particle inspection marks shall be applied on the barrel assemblies and bolts that have passed this test

"This test" is 4.7.4 High-Pressure resistance test.

ETA,

This has been a pet peeve of mine for years. Now, I see that its point is being lost in some supposed "semantic" battle.

The simple fact is this, ANY maker worth their liability insurance will Magnaflux their barrels. Only dutiful companies will run a proof load through them first. This procedure is STANDARD in European proof houses but is not required (except by .mil) in this country.

I have seen barrels that haven't been proofed fail Magnafluxing. The failures were induced by flaws in the steel, the stress of machining, or a combination of those two. There is no point in testing a barrel that hasn't been stressed to a predictable level. That is like checking an empty tire for leaks.

Back to the matter at hand, my reason from the start, oh so freaking long ago, for pointing out the real meanings of the stamps is to give lie to those makers who say, "Oh yeah, we Magnetic Particle inspect our barrels."  The answer to that one is "Duh, you had BETTER at least  Magnaflux your barrels."

Think about it and quit getting bogged down in this tedious bullsiht.

I've spent the evening reading the Subaru forums and I'll be damned if they don't argue the same stupid siht over there as folks do here. I read a four page thread on which is the proper wheel bearing grease.

Quit worrying about the little things (what range to zero at, who made my rifle, what lube to use, what ammo to shoot, ad infcukinfinitum) and worry about the big things; can I hit what I want to? does my rifle work? is my rifle reliable?, do I have the training to make the shot I have count?