Question: As a non-American and therefore with no gun or weapon knowledge, what is the range of riffles or commonly available guns that one could buy off a regular gun shop?
He used an AR-15 which in theory is accurate to about 300m for a proficient shooter, but amateur shooters shouldn't really expect more than 100m. If he hadn't fallen for the marketing campaign he'd have just used a deer rifle and probably gotten the bastard for half the cost.
That all said, a lot of rifles have theoretical effective ranges that only apply to top marksmen, so anything would have worked if he'd had a steadier aim, but that's easier said than done.
My basic training was with an M16A2 manufactured in the 80s and after a couple hours and ~100 rounds of practice I could group center with iron sights at 500y (457m) and that's functionally the same gun. Effective range for an AR-15 should be the same 600y(550m) as the old M16s from the 70s and 80s, extended to ~1000y (914m) with extra training, better sights, and a match grade rifle
What's weird is that I had way more accuracy with the M16 on pop-up targets than I've ever had with an AR-15. But AR's are made by a variety of manufacturers, and I'm sure that some are better/more accurate than others.
Oh, it's not even different manufacturers. Palmetto State Armory has a ton of cheap options, like if you bought a blemished set of their lower grade in 2014ish you could get a full rifle for $400-600, but they also had stuff in the $1500 range that definitely grouped better at 200y (which is the longest distance range I could find that didn't involve giving money to the NRA which at no point am I willing to do)
Edited to add: my main point was that a 30 year old rifle manufactured well shouldn't be that far off from a semi-sloppy modern manufacture, and if the reports that the rifle was stolen from his father are true I'd bias towards a nicer one since the same reports say he stole the rifle but not the sights and was firing without them
The maximum effective range of an AR-15 is about 300m, but that's with training and patience. The average shooter would be happy to consistently hit a 100m target. The Internet will say 500+, but my experience is that it's not possible for the average person.
He probably also aimed for the head, which is way smaller target than body, especially in this case. In military training was mainly to shoot to body, because it really doesn't matter in combat.
Depending on how the gun was configured/chambered, and weather conditions, I think it's quite hard. Also considering he probably only had shooting range experience, and was a kid under heavy stress.
Also not American, but at roughly the same age trained on the HK G3, which uses a bigger caliber and longer barrel, under simulated stress conditions could hit a target at about 400m reliably. After intensive training, and "hit a target" does not necessarily mean a headshot, but somewhere on the body. We were explicitly told to aim for the biggest mass, not fanciness.
I think that shooter was misguided in many ways, and also by advertising, as already mentioned in this thread.