Pre 60's there was amphetamine freely prescribed along with barbiturates, tranquilizers. Not hard to get at all, super common with actors. Half the adults used tobacco and lots of booze.
Caine is a cool dude and all but not really buying his story. There were more types of drugs during and after the 60's, but lots before too.
Meredith Hunter pulled a gun before being stabbed. Jagger was punched by a concert goer. Lots of out of control people there, not just the Angels.
Don't you dare defend the Hell's Angels! Meredith Hunter pulled a gun because the Hell's Angels harassed him all day for being a black man who dared to bring a white woman into their presence. They are nothing but sick rabid hyenas that should be put out of their misery. They will surely rot in hell.
And if you think "everybody riding Harleys today" justifies their hateful, murderous club, then you are just as fucked in the head as them, and you can rot in hell with them