1biggun, I also was wondering how aluminum would work for a GB. Just a machined sleeve for the piston to ride in and the tube to sit on.
Bradrock, I could get a pic of my custom RSB on the .308 if you want. A lot like 1biggun's thinned a little and a Saiga handguard reworked a little to fit together.
ETA: Looking at the 50-110 pic again it looks like it might be a custom GB??? Also is the front sight on backwards? The slope to the front?