I made two of my own not that long ago. The US Berlin Brigade was issued M67 RRs back in the 1970s and '80s as there were potential issues using the M47 Dragon in an urban setting,so I needed a couple for my platoon.
I used the Rubicon plastic figure, one standing one kneeling and with a bit of arm surgery they make good conversion start points. The weapon itself was a thin plastic tube. I carved down a venturi, added a piece of plastic rod underneath as the monopod and another piece of tubing as the optical sight on the left of the barrel.
It's really easy conversion. The only slightly tricky bit is that the pistol grip and firing mechanism is set at any angle on the right hand sight of the barrel. That means aligning the figures right arm at a slight angle. Just a matter of finding a suitable donor arm and slicing it back at the shoulder to a suitable angle.