In practice though, there’s very little (IMO, NO) difference between a military M4 or M16A4 and a civilian AR-15 version, since the military doesn’t fire full-auto anyway.
(source: friend of mine who got out of the Army as a CPT a year and a half ago, and a buddy who is a Marine who got out 15 years ago)
Burst fire is worlds away from semi auto.
The rifles have the capability, but they’re not instructed to use it- it’s all aimed fire these days, even in the Army. I’m pretty sure the USMC never sprayed and prayed.
Ok, but the difference between the two is still thousands of dollars, and either a semi-arduous process or a long prison term. Otherwise, yes, all the accoutrements are mostly window dressing and you can buy “military grade” optics etc.
1 thing not mentioned yet is the super-restrictive nature of some states, that put more restrictive measures than the Federal rules. For example in Cali, they apply the ‘assault weapons’ definition to ban modern military rifles, even tho the Feds have sunsetted this stupid rule. Basically, ‘assault weapon’ means having what they call ‘evil features’, like a handgrip & detachable clip. Even tho they’re semi-automatic, and can shoot no faster than the ww2 relic I own (M1 carbine), the relic is legal & the AR15 or AK47 not. Stupid & arbitrary, especially when u consider that exactly those guns with military experience are those u want in society, adapted for civilian use via semi-automatic. Of course they can be illegally converted to full auto, but so can any gun.
The M4, as used by the USAF, includes the capability for Burst Fire. We’re taught how to use it, and then told to never do so as it’s horribly inaccurate.
Friends of mine in the Marines tell me they are told to never ever use burst fire, instead repeatedly firing single aimed shots at their targets if encountered at close range.
Now, there are a number of military firearms that you can acquire relatively easily in the civilian world, including the M9 (known in the civilian world as the Beretta 92F or 92FS) and the M11 (known as the SIG Saur P228). I’m not sure about others.
A guy I used to know was a bomb loader in the Air Force during the Vietnam War. He said that one day the base was being ‘overrun’. I’m not sure what the deal was, as he was in Thailand. I also don’t know the definition of ‘overrun’. Maybe one guy in the bushes launching mortars? In any case, there was some fast and furious activity and he had to check out an M-16 from the armoury.
Marines were there, stuffing their pockets with extra loaded magazines. The guy I knew said he was issued an M-16… and one round of ammunition. In a sealed plastic bag. That he had to sign for. And he was expected to return it. He said he asked some Marines if they could spare him a full mag, and one almost gave him one.
The story sounds ridiculous, except for the fact that it’s the Air Force and mind-boggling procedural decisions is kind of tradition here. 
That’s only legal in Nevada.
But the Galactic Temporal Police want to talk to you.
It sounds like the implication there is the same as in Pirates of the Caribbean, where they strand you with a single bullet. Or ancient Rome, where they leave you with a blade or poison, and expect to not have to talk to you when they return.