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, *****, 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....