It would seem the actors drew the line at getting a true recruit haircut. Your first haircut and each one you get each week for the next 8 to 12 weeks of basic training, will be nearly bald cuts. Electric clippers are set to leave approx. ¼ inch of hair. The men leaving the barber still have lush hair styles. Something no recruit would ever have.
Sgt. Ryan who is an E-7 salutes his fellow Sgt. who is an E-6 during a live fire exercise. You only salute officers, not enlisted men.
Sgt. Ryan repeatedly calls the barrel of the M-1 rifle the bore. The bore is where you chamber a round.
The bore is the inside of a rifle barrel. Which is cleaned with a cloth patch. The chamber is where you insert the round.
At the beginning of the movie the newly arrived recruits are ordered to do a left face and then an about turn. They wouldn't have known how to perform either one of those maneuvers until later on in basic training.
In the opening Korean War battle scene, an American soldier is shot by an unseen North Korean or Chinese combatant. Only the muzzle of the enemy rifle is seen firing which is clearly a U.S. M1. While some U.S. rifles were captured and used by enemies, it is far more likely to have been a Mosin-Nagant rifle or WW2-era Soviet or Chinese SKS carbine.
When Ryan and Holt have the troops on a training exercise in the field, Ryan tells Holt to "get back to base." The Army doesn't refer to its facilities as bases. An actual soldier would have said "get back to the post."
When SFC Ryan throws the grenade that he set on the sand bags, the resulting explosion is not where he threw it.
At the grenade range opening scene, numerous men are shown throwing grenades but there are only 5 explosions.
After avoiding the MPs outside the club, Sgt.s Ryan and Holt start walking down the street with Julie. A moving shadow of the boom microphone is visible on the wall behind them.