OK, I went through the initial battle scenes in The Terminator almost frame by frame, and I'm still not 100 percent sure about the RSB-80. In one scene they depict the weapon on top of a SUV.
In the photos above I don't see much texture for the barrel support, but in the Franco scene below, the holes in the barrel support are evident.
Definitely a M2HB
Also, the length of the barrel was longer than I initially thought, so I'm thinking that they just slipped a stylized outer jacket over top.
Again, initially I thought that they held the weapon upside down and put the charging plate and handle on the opposite side, but with a hectic shooting schedule, that would be unlikely. The armourers probably just rotated the charging handle downwards. Also, the spade grip in the back looks like it was taken off, reversed, and mounted only on the right side so it probably didn't jab Franco in the abdomen.
I'm pretty certain that the weapon in one of the first scenes was the one Franco used.
The Terminator fandom page has this graphic for the RSB-80 and I'm thinking its reasonably accurate for the general layout. I believe its from a computer game. Below the top cove is removed and a "plasma" magazine or power supply is inserted on top.
However, each time I look at the photos, I see something different. I'm not certain if it is an issue with the screen frames being converted into digital format, or the actual lines of the weapon being illuminated with the flashes and lighting.
But I might as well make a decision on which way to go.
If anyone has another opinion, feel free to mention it. Based on a new outlook on this, I'm going to slightly redesign the shroud.
TazMan2000