In the opening of Full Metal Jacket, Joker gets punched in the guts by Hartman. Could Joker have successfully pleaded Self Defence, had he killed Hartman right then and there?

No.

Almost certainly not, unless perhaps the death part looked like an accident. But striking a superior officer in the military, or even disobeying a senior NCO, are serious offenses and they can lock up recruits just for that if they want to.

A sergeant hitting a trainee might be brushed off as training… but times and practices have changed. Here is one article from 2005 about less savage training tendencies.

No. As @Zaku said, that could end up with the recruit going to jail. Plus, that was how they trained the men in the Vietnam war. The movie is pretty accurate in a lot of ways.
A lot of the men were forced and didn’t want to fight, so they resorted to other ways to break them down. Now, many people willingly enlist so such measures usually aren’t necessary
Although, a Drill Sergeant was just arrested for hitting recruits during training. Just recently. I’ll try to find an article

My father was in the Vietnam War. He was also a drill sergeant for years after he recovered from injuries.
My understanding is that such things were quite common. And retaliation would normally end with the private getting their butt kicked,by the sergeant.

The sergeant, in many cases, tries to create a situation where the troops bond over their distaste of their drill sergeant. An army is a chain. Only as strong as it’s weakest link. In this case, the sergeant knew that Pile was the weak link. He put it on the rest of the troops to pull him through training. Just as if helping a wounded soldier, in a real scenario. They must work in unison, and take up each other’s slack.

Behavior such as seen in FMJ, was eventually frowned upon. Sweeping changes were made. Now, they aren’t supposed to touch the troops, and can be reported and/or punished…

In the time that FMJ was taking place, Pile had no choice but to pull through. If it were earlier in the war, he would have been sent to jungle training, after basic training. So, he lucked out there. But, as we know, (spoiler alert) Pile spiraled down the steps of insanity…