You want to heat it enough to soften the thread sealant(to the point that it smokes). But what you really need is a tool that is used for removing m16 flash suppressors as well as a spanner that fits the nut, unsure what it is for m16, 3/4"? Also i think 15-22's are left hand thread? However, as danf_fi said, if it is going to make the rifle undersize for the laws of your country/state, its ill advised.
"Sir we are surrounded."
"Good! We can attack in any direction."