There is no such thing as an assault rifle, so the premise of the negotiations is false.
Almost certainly not. Trump either knows or would quickly come to realize the left calls most any weapon that is mag, drum or clip operated an assault weapon whether or not it is full auto.
Further, bear in mind that most of the left believes the Constitution is fundamentally flawed. It has been said that negotiating just about anything of significance with the left suggests you would be willing to have some amount of arsenic in your water.