The FGHP is from Traveller, it deals a shedload of damage (2d6 times ten in a system where the average person has 21 'hp'), but has the unfortunate side effect of venting radiation all over everything when you fire it. It's meant to be used in conjunction with a pretty good hazmat suit, but those can get expensive and a cheap enough hireling could run cheaper...

Yeah, there's a heck of a lot of overlap between Traveller and 40k- they were developed around the same times (classic traveller was '77, Megatraveller was '86, Rogue Trader was '87 and then Traveller: The New Era was '92) so there's a lot of cross-pollination between the two