There's no such thing as an "Assault Weapon". Purely a subjective political buzzword crafted in the 90s to make black finish guns like the AR-15 look scary, when the reality is that there are plenty of other guns that don't fall under that nebulous category, that have identical "deadly potential". The garden variety AR-15 is merely a semi-automatic rifle, like the SKS, or Ruger 10/22, the only difference is that it's scary looking to the naive and feeble, because it has 60 year old modernizations, such as magazines, instead of stripperclips, and some level of modularity.
but I'm a bit more skeptical of shotguns, sniper rifles and machine guns.
Why? Shotguns are used for hunting, have very limited effective range, and most non-magazine-fed ones take forever to reload. Never a good choice for a school shooting.
As for "Sniper Rifle", that is, once again, not a real term. It's a good colloquial for the normies, so they have a name for "gun with a scope on it", but the term you're looking for is Designated Marksman Rifle. At the end of the day, what you're skeptical about is the fact that someone can put optics on a rifle, and you can literally do that to anything. Hell, I once put a 4x Marlin scope on a pistol before, does that make me a sniper now? Again, not a common killing spree weapon, because, surprise, you actually need experience to snipe someone. Most guns that would fall under this category, once again, are extremely common hunting rifles, but with optics. The firing rate is about as fast as you can flip the bolt back and forth. If you wanted to ban "Sniper Rifles", you would have to ban ALL rifles.
And when it comes to machine guns, the only way you can get one in the U.S. is through the ATF tax stamp, which means you have to pay a $200 fee on top of the over $15k you'll be spending on a pre-1986 fully-automatic weapon, which you probably won't be approved for anyway. Even if by some miracle you manage to clear all of the hurdles, you essentially sign away your reasonable right to privacy with the tax stamp, because you're giving the ATF consent to check up on your gun at any time without a warrant. If you own an M249, and someone goes on a killing spree with one in a 50mi radius of you, you will get a knock at your door. Nobody goes through all of that rigmarole just to shoot up a school, when you can just talk to your friendly neighborhood former Soviet, or Vietnam vet, and get something that way, totally under the radar.
I think it's kind of funny that it's a LOT fucking easier to get guns illegally all over the world than it is to get them the right way. Most people don't think that's the case, but it's usually because those people don't try. Five years ago, I knew a guy who had crates of hand grenades for sale.