I was using it as an example. It can be any rifle with a high capacity magazine, and intermediate (or full powered) ammo. It's just that the AR-15 platform is more widely available than, say a SCAR (as you can build a cheap one for 500-600 bucks). And i'm not talking about milsurp like garands and SKS
And for the counting shots and magazine change: You have a point, but still, that 2 secons becomes 6 or 8 when you try to shoot 40 shots, instead of 2 (3 magazine changes instead of 1), and that's considering that you are well versed and well trained in the manual of arms of said weapon(s).
And for the point on mental health care. Yes, but the GOP has refused to pass laws regarding them. They only use said problem as a scapegoat, in order to ignore the other problem (that guns and ammo are too easy to get in some states).
And mind you, i'm not against guns or that banning all intermediate or full powered cartridge rifles is a good idea. In fact i hope that with the new constitution in my country, getting said weapons becomes legal again. I'm just saying that it should be specially hard (as in, time consuming) to get that, and to get ammo.