You can legally own a M203 or a M79. Just fill out the forms and pay your money and wait. But all the fun rounds are banned. You can get chalk, CS and smoke rounds.
I prefer the M79 to the M203. Grenades are very important and a dedicated platform is superior to a dual purpose platform imo. I never felt that the M203 was accurate enough. A M79 grenadier can always carry a rifle too.
All of the 40 mm round now are classified as a low order explosive when you have to have an explosives license now
As I understand it, there were X number of “fun” rounds registered on the destructive device list with ATF, and the manufacturers have since bought up most of them.