37mm is the current legal limit. 

40mm is considered a "destructive device" currently. 

You can buy hand grenades and I think even RPG rounds, but, you need to be quite fiat rich as you basically have to pay an additional $200 stamp per device, plus you'd need to be licensed by the ATF and usually your state level gubment. 

You can, of course, roll your own, but, for me, having sufficient variety of firearms and ammo is more than enough. If a company sized element comes for me, I'm pretty boned and will likely be overwhelmed no matter what I do.