There are already plenty of types of guns that you cannot buy (machine guns, grenade launchers, etc.), and that seems to be fine and legal. We're just talking about adding more to that list.
>How far do you want to go? P90s? RPGs? Stinger missiles?
Silencers/supressors are currently banned in the US (requires a tax stamp and long waiting period) but legal in other highly restrictive countries like England/UK. They're basically mufflers for guns and not like in the movies. Short barreled rifles would be nice too; we have those now but it's a "technicality," where anyone who owns them for recreational shooting can, on a whim of a bureaucrat, become a felon sentenced to decades of prison. That's not how a free society based on laws should work. Pot falls under the same category. It's still not legal at the federal level and if the DEA decides it wants to start raiding people's homes looking for pot, it certainly can, and people can be prosecuted for it.
>What would the societal benefit be there?
We're a society of individuals, not a society of Borg. Freedoms of the individual are the greatest benefit a society can have.