I know exactly what you're talking about. It came in a role-playing set that Hasbro/Kenner put out years ago. It included a horrid one piece armor plate, an overly small mask/helmet thing, a couple of gauntlets, and the gun. I bought a set of that when I was 14 or 15 and stuck the gauntlets onto some dixie cups I painted and used the blaster with the wrest of the costume that I had made.
I can't for the life of me find it online, but I am pretty certain this is exactly what you saw, sans the rest of the set!