From this thread
https://www.ar15.com/forums/ar-15/Bushmaster_Barrel_Marking_/118-219702/ Since the 80s, all military barrels are stamped with some variation of what you see below. BM began marking their barrels like this when they got a small one-time supplimentary contract for M4s during Desert Shield, and were (illegally) given (by the Army) the Technical Data Package for the M4.
B MP 5.56 NATO 1/9 HBAR
B = Bushmaster (this varies by manufacturer, of course. C = Colt, FNMI = FN, etc.)
M = Magnetic Particle Tested (mangnafluxed)
P = Proof Load Tested
5.56 NATO = Chamber dimensions, as opposed to the tigher commercial ".223 REM" chamber
1/9 = Rifling Twist Rate. 1 turn every 9 inches. Military barrels are 1:7, or (older) 1:12.
HBAR = Heavy Barrel, and only seen on HBAR-profile barrels