Grasshopper, you have been fed at least part of a load of ca-ca.
First, look up the term "Destructive Device". The WW2 Bazooka, which is a rocket launcher, and it's larger replacement- the 3.5 inch rocket launcher- are destructive devices. They are treated much the same way that machine guns are treated. The M-72 LAW is not such a device- once it has been fired. Since it is not reusable as a weapon- one shot and it is done- the expended launcher is not a DD.
Each round of bazooka ammo that has an explosive warhead is also a destructive device.
Now, with grenade launchers, there is some difference. The military uses a 40mm, either the M79, the M203, or one of the belt feds. Again- destructive device. Each round of EXPLOSIVE ammo- destructive device.
HOWEVER- there is a 37mm Gas gun- for which explosive ammo is not made. Called a gas gun because that is what the police dept uses to lob a tear gas round thru your window. Those are not destructive devices when used for gas, flares, smoke, etc. There is also a 26.5 mm Flare launcher that is not a destructive device.
Now, someone asks- what happens if you take a 37mm smoke round, open it, pack it with tannerite and a blasting cap? Well, that is really simple. Snake tells you that you had better getcher azz back to the cell, bitch, and get the laundry done. (10 yrs, Federal prison, violation of 1934 National Firearms Act, no parole, and a $10,000 fine).
EDIT_ sorry- you asked about RPGs. They are also DDs, Each round of ammo with an explosive warhead- DD. Two years ago did an investigation of an accident at a research facility working with a version of RPGs- and trust me, those are rather rigidly controlled. There is a TRAINING AID that is a dummy launcher that is not a DD, but it also cannot shoot rockets 960 meters.
But as he said right up there ^^^- if you would like an M203 for FREE, just go stand over there. When I tell you, raise your right hand, say "I do" and take one step forward. We can get you fixed up with new boots, a haircut, and a place to shoot. And a great FREE exercise program....