I am a member of the National Rifle Association. I don't remember when I joined, but it's been a while.
I own a 9mm Barretta. It is locked in a box separate from its ammunition which is also separated from the key that opens the box.
In the days when I spent some significant amount of time in places like Afghanistan or Sub-Saharan Africa, I would take my pistol to the NRA range and polish up my skills before I left.
Just in case.
I never needed to fire a weapon on any of those trips, but that's part of the point: I don't need a 9mm Barretta, but I like to have it, I like to be able to fire it, and I like to increase the odds that I will hit whatever or, God forbid, whomever I might have to aim at.
Do I need 40+ pistols or long guns? No. Nor do I want any more. But some people do. Gun collecting is a legal and, for those who are into it, an enjoyable activity.
As to assault-style weapons, I am not so sure. An AR-15 is the civilian form of the standard military rifle, the M-16. An M-16 is selectable - this is, it can fire one round at a time (semi-automatic) or in a burst (automatic) mode with a flip of a selector switch.
The civilian version has no selection for automatic fire, but I believe someone moderately skilled with working on guns might be able to do that.
I say that because when I was on active duty, I was trained as a "Unit Armorer" at the Quartermaster School in Ft. Lee, Virginia.
I fixed everything from a .38 pistol (still a fairly common sidearm in the 1960s) to a .50 caliber machine gun.
SIDEBAR
There is nothing - no part of a .50 caliber machine gun - including the ammunition, that is not heavy.
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M-16s were still relatively new as the standard-issue rifle, having replaced the M-14 shortly before I went into the National Guard. In fact, when I first joined, my unit - part of the 50th Armored Division in New Jersey - was issued M-1 rifles - just like the ones the Band of Brothers used in World War II.
There were some problems with those early M-16s. The part that allowed it to fire in semi-automatic mode would occasionally wear out, or break, and the rifle would ONLY fire in automatic mode. In fact, it would keep firing until the ammunition was exhausted.
It would not, I think, be impossible to reverse engineer that process.
I am not an expert in gun laws. I think that gun shows or other private sellers should have enough computer (or smartphone) capability to do an "instant background check" on a potential buyer. If I sell you my car, I have to let the Commonwealth of Virginia know that I no longer own my car and you have to let your state know you now do.
If I want to sell you my Nine, I should have to show that I ran your name through a website to prove to me you are allowed to own that weapon - or, at least, there is nothing to show you can't own that weapon.
I can own any number of vehicles, but if I want to drive them on a public road, they each have to be registered.
I know there is no Constitutional right to Keep and Bear Cars, but you understand what I'm saying.
I absolutely believe that being a 2nd Amendment guy does not mean I can't argue for some changes in the law.
Would they have prevented the Las Vegas massacre? Probably not, but we'll never know.
Gun ownership is a Constitutionally protected right. But, just as falsely shouting "Fire" in a crowded movie theater isn't protected by the 1st Amendment, it is not a protected right without limit.