There are something like 300 million guns in the US
Maybe more. Something like 1/3 of all households have firearms in them.
Semi-automatic weapons have become so popular in the past 40 years that they now overwhelmingly outsell all other guns for all purposes, such as hunting, target shooting, and home defense. I've read estimates stating that the MAJORITY of guns in this country are now semi-automatic. People are buying 9mm's instead of .357 revolvers for home defense, semi-auto 12-gauge guns instead of pump actions for duck hunting, semi-auto .22's instead of bolt-actions for squirrels,rabbits, and target practice, and semi-auto AR-15 style rifles for instead of lever-action .30-30's for deer and bear.
An attempt to ban all semi-automatic weapons would be a political clusterfuck of epic proportions. You'd not only have to deal with the fallout from the few million hardcore gun owners with arsenals of assault rifles, but the tens of millions of pissed-off homeowners and hunters who would be losing their guns as well.