DJRJ,
Didn't Mega Traveller have belt fed energy weapons?
They were chemical lasers, similar to our chemical laser planes today. Instead of semi-truck sized vats of highly caustic/corrosive chemicals, the weapons had belt fed chemical batteries. Still not environmentally friendly though...
Then this tech was used for the "plasers". The weapon had a laser that heated a hydrogen pellet contained in a magbottle and then pushed the super hot thingy from the barrel.
This weapon doesn't seem viable w/ hydrogen, but it did massive damage if I remember.
Maybe "foam phased metallic hydrogen" could work. When under gas giant pressure, I think it acts as a metal...
For less tech, a smaller version of a laser heated railgun might work. Don't have to worry about the gas quickly dispersing as it's almost liquid metal...Splash damage on nearby troops or more than one random crit.