In my state (it’s different in each state) you can buy “long guns,” at 18, with a NICS background check. At 21, you can buy handguns, and most others. Again, with a background check. Yes. An AR-15, is considered a “long gun.” No increased scrutiny for large purchase.Really. You can buy 100 assault rifles, and 100,000 rounds of ammunition, and no additional attention would be focused on you. At some gun shows, no background check is required. You need an FFL (federal firearms license) to sell them. But. There are no laws on private sales. So. If I met a stranger at a shooting range, and he said “nice gun, can I buy it from you?” I can sell it to him legally. No background check required. Guns have serial numbers, but it is up to the owner to keep track of the number. So. If a gun was found at the scene of a crime, there’s not really a way to track it to it’s owner. There are no restrictions on ammo, except that you have to be 18 to buy rifle/shotgun ammo, and 21 to buy handgun ammo. There are a large amount of aftermarket parts, upgrades available, that require no background check. Large capacity magazines are easy to find, for many weapons that have an external magazine. THE single biggest problem is that there are hundreds of millions of legal firearms in circulation. That is the biggest difference in circumstance, between the US, and the rest of the world. Most states have a variation of a legal concealed weapons permit. In Texas, you can just walk around with a rifle, or whatever. Some of the neighborhoods that I’ve lived in, there are people walking around with pistols in open sight. Probably illegally obtained, and probably illegally carrying. I say probably, because many are under 18 years old, and some are known felons… If you have any specific questions, I used to have a FFL, and sold guns for about 3 years. Maybe I can help…