Anything short of Guided MIssiles, really. I am going by what it is here in Kansas, which has a much lower murder rate than the US at large and could give some Western European States a run for thier money. And what is it here:
1) Conceal Carry is "Shall Issue", meaning if you apply for a carry permit and dont have anything on your record that would disqualify you, the state MUST give you a permit.
2) Full Auto is completely legal. In fact, you can even buy some newer than 1986 thanks to the Commerce Clause (althought these are usually reserved for gun ranges since it can be sure this gun will not leave the state and violate the Commerce Clause).
3) Guided Missiles are Illegal, but Unguided Rockets are not. Meaning you cant have a stinger missile, but a Bazooka or RPG is A-ok. However, each individual rocket must be registered (rather than the firing device) and you must meat the same rules as explosives (see below).
4) Anything firing a round less than 40mm in diameter is legal, excluding rockets (see above). However, if the round is explosive (like a 40mm grenade), then again each individual round must be registered.
5) Tanks and other military vehicles are legal. However, if the cannon is over 40mm, it must be decommissioned as per state law (the breach must be welded shut in a state police facility), and you still must meet weight limits on local roads. In addition, while wheeled vehicles can be driven anywhere so long as they are light enough, tracked vehicles must be moved on a trailer and can only be driven off road on private property. And assuming you have a place to keep it and take off, retired military aircraft are legal too, but these must be completely disarmed even if their guns are otherwise legal under other laws. I dont know if you can have retired military jets (though I would be fine with people owning those), but there are plenty of people out there with old WW2 aircraft they fly, ranging from simple trainers to B-24 bombers.[footnote] There is also a guy who owns the only flying B-29 left, but he is from another state.[/footnote]
6) It is against state law to require anyone to have a permit to own a gun, as well as being illegal to force people to take safetly classes before owning a gun. This might stem from the fact that around here people learn guns from an early age, and will know how to use a gun by the time they are at least 13, so the state says "Why bother, everyone already know how to use them."
7) Explosives are a bit bizarre. You can buy most explosives (although fireworks are restricted), and you can build your own homemade ones, but there is a catch: You cant build or use them inside urban areas, except on specific days, like the 4th of July. If you want to make explosives, or set them off, you must have 20+ acres of land, the landowners consent (and if you ARE the land owner, it makes it that much easier), and permission from the local fire department (which will usually be a rural volunteer force).The Fire Department will give you the go or no go. IE, if there is a high grass fire danger that day they will forbid you from setting them off, and if you do anyway, you will be held criminally responsible. This applies to control burns as well.
And yet with all that, we have a murder rate of 2.1 and dropping, compared to the US average of 4.2 and dropping. California is the Gun Nazis, and yet they have a murder rate MUCH higher than the rest of us. So call me crazy for thinking guns arent the problem.