This would launch a cartidge (sp) with methane in it. I'm sure u know that if you breathe in all that methane your lungs fill up with water and you drown. It would have about the same range as the explosion of a dew grenade and would decipate after a few seconds.
advantages
you can put out nearby napalm
drown multiple enemies in one shot
disadvantes
you can easily drown yourself...
limited ammunition
Gas mask
This would be like flame or blast armor, except it would protect against the methane. I think thats all that needs to be said here.
EDIT: My bad, methane robs things of oxygen. So it would still have the same effect on both targets and napalm, it would just suffocate instead of drown.