Hi Rob, I know these are held in place with some kind of compound coating, at least that's how the factory does it. In that case the word is to heat it, and with a BF wrench and perhaps a hammer break it free.
What would trouble me about this on your rifle is this appears to be something someones done aftermarket, and god only knows what they've put in there. People do crazy and sometimes dumb things to their guns. I would really take this to a gunsmith to have it removed.
Perhaps your skills with this kinda thing is higher then mine, but I'd be afraid of- 1. using too much heat and damaging the barrel, and 2. beating the hell out of it and perhaps damaging the barrel that way.