well im not 100% sure but some how butane seems like a bad idea..
altho the other gasses are also flamable butane is really flamable so a small spark or static discharge while fireing it may cause a small flame ball in your hand..
i dont think there would be to much damage by this quick flare up but if it happened on a cold day when the gbb has the chance to stay open and vent all the gass then yes that would be an issue
but thats just my 2 cents
*edit* oop.. a bit late i guess thats y..