So I signed up just to reply to this, I got the same glitch on my PS3 and could not get a companion to pick up the gun. However, if you are careful and blow a few grenades in just the right spots behind the capsul a corner of the gun wound up just barely selectable. It took 3 or so grenades to get it to bounce right, first lying flat then another one behind the capsule nudged it so I could fish with the pointer and pick it up. Hope this helps.
Thank you so much! I had signed up just to post this glitch. I had tried the explosives before, but had no luck. Your post inspired me to try several more times, and I finally got the gun.
Thanks. For others, my experience was that placing a mine on the left side of the capsule, shooting it, and repeating, worked.