I worked for the DoD on a project in Iraq and saw some interactions between Special Forces groups. Spent a lot of time with Rangers from Hunter AAF in Georgia. I think this scene employs a bit of artistic license to help set up story points later in the film.
To the heart of your question, I'd think a Delta Sergeant would pull him aside and have him flip the ...
In case you missed it, the scene starts with the troops having an impromptu barbecue with the wild boar killed by Hoot.
Hence, the "aerial practice" Hoot is referring to is the act of killing the beast.
"Didn't want to leave it behind" is for the dead animal itself.
The short answer is Hollywood. The M16A varieties have a mounting hole in the carrying handle.
During the time of this event, both units would have trained with iron sights. Soldiers in units with less regimented structure have more leeway to allow their soldiers to outfit their weapons with more customized gear.
One advantage of standardizing the individual ...
This was in the early 90's. Back then scope distribution among all troops was not standard issue for rifles back then. Picatinny rail was still a few years away. SOME of the Rangers in fact did have some scopes. They were just attached differently.
I'm going to take a swing at this and say, it's because the costume designer & props made a joint decision, most likely in conjunction with the director, to provide this difference as one of the aids to the audience in more rapidly identifying who was with which unit.
As everyone is wearing approximately the same colour uniform & faces are not ...