Only Level III and Level IV plates can stop rifle rounds of any kind. These plates are made of rigid materials like steel or ceramic materials blended with aramid fibers and bonded with resin. The resulting material is a hard but lightweight plate capable of defeating intermediate rifle rounds at Level III such as the 5.56x45mm, 7.62x39, and 7.62x51mm NATO, while Level IV plates are rated for larger caliber rounds such as the .30-06 AP, also known in the military as the M80.
it’s also possible that Doc Brown whipped his own armor.
That is a part of the problem; there are a lot of ‘coincidental’ things that Marty does—such as introducing Chuck Berry to “Johnny B. Goode”, convincing the diner employee to run for Mayor—that seem to be part of a consistent chronology, but then he also does things like convince his father to embark on a successful writing career (or at least self-publish his novel), causing Biff to become a cowed sycophant, gets ‘Mad Dog’ Tannen arrested, et cetera. This all works out well for ‘Prime’ Marty within the convoluted timeline of the film but as cycles go on and changes accumulate there must be a sequence of events that go out of control, spurring on nuclear holocaust or fomenting fascist revolution in the United States, which at some point eliminates ‘Doc’ Brown and prevents the development of the flux capacitor, which would end the perturbation of the timeline and result in a dire future where everything is worse. The only way this could be stopped is if there is someone outside of this entire world-line who is constantly intervening to prevent the worst from happening, i.e. Eric Stoltz and Melora Hardin from being cast as Marty and Jennifer.
Back To The Future is evidence of a god. Not the God, but a god. One who works in mysterious ways as an anonymous Hollywood film producer who never leaves his home in Punxsutawney, Pennsylvania.
