OK, what do I need to know? I see used US military kevlar helmets available. I assume these new style helmets can deflect bullets and shrapnel better than the old "steel pots" I had when I was serving (which were not that great for deflecting bullets, BTW)......so, how well do they protect? I see some marked M1, M2, M3, etc.....are some better than the others? How are the sizes calculated? I wear a Medium Harley helmet.....does this indicate a Medium as far as the US military calculates it? Give me some input here.....because this is an initial inquiry, and I have absolutely no idea what I'm talking about..... I see some others marked level IIIA, which are really expensive.....claim to be bullet proof. Most of these are in the $300 range, and I'd rather not pay that much.