Because you're already for banning certain guns. In 1986 we decided full auto weapons could no longer be manufactured and the ones already in existence became HIGHLY regulated. If you wanted one you had to pass a rigorous FBI background check and your local police station was notified you had one. Guess what, one has never been used in a mass shooting. Now, do you ever hear, even the most die hard NRA devotees, complain about it? No. Essentially we all agree some weapons shouldn't be in the possession of average joe citizen without some serious oversight.
So what's the big deal about putting high capacity semi auto weapons into that same category? Just like after the 1986 legislation, no one is going to go after your pistols or "regular" hunting rifles or shotguns. The 2A isn't going anywhere, nor should it. So why do people freakout like it some sort of tyranny to stop producing guns like the AR-15 and regulate those in circulation like the full autos?