A semi-automatic rifle without a large-capacity magazine is useless as an assault rifle. A fully automatic rifle fires around 700 rounds a minute. To put this into perspective, this is a little over 11 rounds a second. If I had an automatic rifle with a clip which held just ten rounds, the clip would be exhausted in less than one second.
A semi-automatic rifle can be converted into an automatic one by one of two ways:
The first requires filing a certain ratchet pawl, so when the carriage returns with a new round, it skips stopping for the trigger, and fires again instead, and continues to do so, round after round, until the clip is exhausted. Now the rifle is not only automatic but permanently so.
The second requires a "bump stock", which I presume to be a hollow stock with a sort of shock absorber inside. The part of the stock which goes against your shoulder is attached to a piston rod which goes inside the stock. It rests against a spring inside the stock. The spring will throw the whole rifle about an inch or so beyond were it usually rests, once the rifle recoils, temporarily lengthening the weapon. A string or wire can then have one end attached to the piston and the other to the trigger. Once the recoils lengthens the weapon, the string or wire pulls the trigger. This system probably doesn't have the fire rate of a true automatic, but it can fire at a far greater rate than a semi-automatic. The beauty of this system is that it is very easy to convert the weapon back to semi-automatic. To do so, one merely has to jamb the piston, so it cannot move within the stock--or merely remove the string or wire from the trigger.
So the real threat to public safety is not so much the rifle but the large-capacity magazine attached to it. This alone should be banned. Otherwise, all semi-automatic rifles will have to be banned, including those used for hunting. Semi-automatic pistols would also have to go.