While I have little problem with firearm ownership, I don't see what it has to do with 9/11.
Unless they are now allowing American citizens to buy SAMs.
This.
9/11 + gun control = 
I think the argument is that if you allow people to carry guns on a plane, the hijackers would have been killed.
Mind you, there would have been about 500 holes in the plane, probably the pilots would have caught a few, and even if they didn't, most of the passengers would have been killed in the world's largest "Polish firing squad"...
...but there's probably a downside.
You see, I thought that may be the case, that someone was suggesting handguns on a plane could stop it being hijacked, without it being thought through.
I also thought that if you had 4 hijackers and the element of surprise, you could also probably kill enough people to keep the rest in submission, so long as no-one accidentally mentioned the whole "suicide mission" thing, since they could also presumably carry guns onto the plane. As I recall, the passengers only fought back against the hijackers once they found out other hijacked planes had been crashed into the Pentagon and WTC, otherwise they thought they were in for a terrifying 48 hours, then commandos storming the plane and doing the dirty.