How about an old school african big game rifle of insane proportions?
My group had run into a hunter who managed to get himself killed on a illegal hunt on a restricted world...no, my heroes did not kill him...they did loot his ship though!
One of the toys they grabbed was a gun case made of exotic woods and precious metals...but very huge in size...they grabbed it and ran back to their ship to get while the getting was good.
5 weeks later they asked me what was in it.
I described a truly beautiful work of art...rare woods...chasing...inlays of ivory...it was one pimped out hunting rifle...
Then they registered what I had said the caliber was...ONE INCH.
Yep, that baby chambered a 25mm cannon round, was semi-automatic, and had a 5 round box magazine...and one of the biggest optical sights they had ever seen.
They asked me just what in blue blazes did you hunt with that!!!
I merely replied that they never checked when they looted the safari ship to see what he was there for, or, what/where he may have hunted prior to his death.
This brought about one of the worst bouts of paranoia that has been my delight to watch as a referee. They imagined all sorts of horrible monsters...each worse than the last...
When I asked my merry bunch of gun junkies who was going to fire it, my father who was a usmc gunnery sgt and rifle range instructor dropped in and told all kinds of war stories about what that "cartridge" would do to your body...
No one, and I mean no one, has ever fired the 25mm Kain Tuck Long Rifle...
I never told them the damage [or had to come up with what it would do] because no one ever fired it. I have lost its specs sheet long ago much to my grief.
But...mounted on one of my heroes trophy room walls is the only gun to have scared the living drek out of my players,