I don't disagree, but question: does someone need to defend themselves with an assault rifle or a submachine gun. A pistol, or even shotgun I can understand...
Most AR15s are used for target practice or small game hunting, the only ones you can buy fire at the same rate as a pistol with a smaller bullet. Just to give you an idea on its bullet size... .22(most common), .223 (AR-15), .380(9mm (2nd common)), .40, .45 (third most common). The AR-15 has more energy, more power at range, but tiny bullets. The .40 and .45 are twice the diameter and weight of .223. Machine guns of any kind are already illegal, so not worth addressing.
I've seen tests with small capacity magazines and it had absolutely no effect in reducing the shooters performance for the most part. So just because an AR-15 has a 30 round mag and a glock has a 17+ isn't going to make much of a difference. They sell up to 30 round for glocks, and the bullet is larger/fire rate is the same. Theoretically, the glock in close quarters is more dangerous.
Shotguns do extreme target damage, much more than pistols but have the added problem of spraying ammo all over the place. They also cannot be operated with one hand, which is potentially an issue. People do not "get up" from torso up shotgun wounds, I think they're overkill and much deadlier than the AR-15 or pistol for most purposes. They make gruesome and gory wounds, not clean holes that a doctor can fix.
The short of this is, the liberal position on guns is not founded very much on the realities of the weapons.