I would really like to understand

[quickesst]

2. A couple of thoughts..

You have to admit that all those things combined make for a pretty formidable weapon since the type of weapon you are referring to, or at least some of them are able to be altered easily into a fully automatic machine gun. I did it to my Chinese made SKS rifle with a 1/8 in piece of metal bent and shaped into what's called a sear pin. Of course this has been many years ago and the SKS and I have a long parted ways.
My only other thought is on magazines. If there is a way to detach a magazine from its rifle, the capacity will only depend on the maker of said magazine. On the other hand, a built-in receiver such as on a shotgun or Winchester 73 which only has the capacity of a set amount of rounds due to size limitations would deter any alteration turning it into a mechanism designed and built for war. A hunter only needs one round to kill a deer if he's good, two if they're average, and the third shot is usually at a target that is no longer there. It's apparent how ridiculous a 30 round magazine sounds. With limited, unalterable ammo feeds, there's a possibility the loss of life can be greatly reduced, as opposed to the loss of life given the capacity and possibility a 30 round magazine offers.
Like I said, just a few thoughts.