I heard it said that it was pretty authentic. what was and what wasn't accurate?
A relative question. For Vietnam-era boot camp? It was probably pretty accurate minus Gunny Hartman pushing Private Pyle over the edge. R Lee Ermey, who portrayed Gunny Hartman was a Marine Drill Instructor and he played the role to a "t". He was exaclt how I remember my drill instructors, and as a drill instructor, we all played the same role.
The constant, every-minute jammed with training and/or running from one type training to another is pretty accurate. The training schedule in Marine boot camp is God's Law, and only an act of God will cause any deviation from it.
The physical abuse as far as drill instructors putting their hands on privates was a bit much. Gunny Hartman was like everything that could go wrong with a drill instructor and he played the wrong role. The Senior Drill Instructor is NEVER the heavy. The SDI is "daddy." The junior drill instructor is usually the heavy.
When you make the SDI the heavy instead of "daddy", it gives the recruits a sense of hopelessness as if they have nowhere to turn because the guy who is supposed to be handling their issues is the one tormenting them. It's a morale killer.
As an senior, Hartman had too much "hands on". The senior is pat of the drill instructor team; yet, a little bit aloof from the other hats to create that separation recruits pick up on whether consciously or unconsciously. In Full Metal jacket, the junior drill intructors had minimal presence.
Where a difference I can't address exists is that Full Metal Jacket was a Vietnam-era scenario and recruits were drafted. I was a drill instructor in the all volunteer Marine Corps. Little short of death got you out of boot camp during the former; while, during the latter, one merely had to refuse to train and would be eventually administratively discharged. That alone would completely change the mindset of the drill instructors.
Oh, and while I won't say it never happens, a drill instructor that strikes a private has failed. He has failed as a teacher and as a Marine. He destroys the respect that recruit has for the uniform and the title "Marine" and definitely brings dishonor upon himself and the Corps. Recruits are at the drill instructor's mercy completely. Their is no excuse for abusing that power.
My two cents.