First there is the problem that the two types of weapon are functionally different. A fully automatic weapon and an "Assault Weapon" (which is defined almost solely by cosmetic features) are two very different things. Even a fully-automatic weapon (such as a machine gun) and true assault rifle (select fire weapon) are functionally very different.
Second is the idea that one supports the full-auto ban in the first place. We'll come back to that later.
Leftists would have you believe that semi-automatic weapons are, by their definition, more deadly than weapons that require some sort of "action" to chamber the next round. (That's what "bolt-action" or "lever-action" or "pump-action" mean- you have to physically manipulate the gun to get a bullet into position for firing). So, by this logic, this is an ultra deadly weapon that no one should own:
|A Highly Deadly Semi-Automatic Rifle. In .22LR
Where as this is a nice safe weapon, that doesn't fire at the rate of one pull = one bullet:
|A Nice, Safe Howitzer requires reloading between shots.
Hmmm... maybe it's not the frequency of bullets being fired that makes a weapon dangerous, but the size of the round.
In that case this:
|A .45 Revolver Fires a Big Bullet
is more dangerous than this:
|NATO 5.56 is not much larger than .22 caliber.
Wait. That can't be it...
No. What they really want to ban are all weapons that scare them- and all weapons scare them.
Now, as to supporting the full-auto ban in the first place. Some people do. I don't.
The point that 2nd Amendment Advocates try to make is (primarily) this: responsible gun owners are responsible. The weapon is not, itself, dangerous, and people who are going to be a danger will find a way to be a danger no matter what weapons are illegal. I don't see how that fails to apply to a machine gun, a howitzer, or an M1A2 Abrams Main Battle Tank. Certainly we should hold people accountable for their behavior- and mishandling a weapon (any weapon) in such a way that it injures or kills someone else should be (and is) a crime. But if the point of the 2nd Amendment is to protect the People from the depredations of a tyrannical government (it is), then there is no philosophical basis to prevent honest, responsible gun owners from purchasing and owning any weapon they desire.