Chaplin said:
It depends on what you would call "experienced". He was a military advisor on Apocalypse Now, and Coppola gave him an uncredited role in the film. That was 1979. In 1978, he was in a movie called The Boys of Company C, in which he was--you guessed it--a drill instructor. That was his first film. Then 5 years after Apocalypse Now, in 1984 he was in a movie called Purple Hearts, and then there was Full Metal Jacket in 1987. So he wasn't really that experienced as yet.
My take on him was that he actually was playing a character very similar to what he was in the military, so that's why his performance looked so natural, not because he was a seasoned actor at the time.
from imdb.com
A talented character actor known for his military roles, R. Lee Ermey was in the US Marine Corps for 11 years. He rose to the rank of Staff Sergeant, and his service included 1-1/2 tours in Vietnam. After injuries forced him to retire from the Corps, he moved to the Phillipines, enrolling in the University of Manila, where he studied Criminology and Drama. He APPEARED IN SEVERAL FILIPINO FILMS BEFORE BEING CAST AS A HELICOPTER PILOT in Francis Ford Coppola's Apocalypse Now (1979). Due to his Vietnam experiences, Coppola also utilized him as a technical adviser. He got a featured role in Sidney J. Furie's The Boys in Company C (1978), playing a drill instructor. Ermey worked with Furie again in Purple Hearts (1984). However, his most famous (or infamous) role came as Gunnery Sergeant Hartman in Stanley Kubrick's Full Metal Jacket (1987)...
The guy had been acting for over 8 years. That makes him an experienced actor, especially considering that new actors (like Emery was out of the army) are constantly working on things you've never heard of before or ever heard of period during their struggle to make it (you know that as well as I do Chap).
As far as your take, your probably right, but it doesn't take away from the fact that the guy was an actor and had been acting for 8 years previous to the film - it wasn't like they just dragged him right out of the Corps and threw him in there, which it seems like people are suggesting.