It was not cheap (I want to say maybe $350 but I cant remember) but this guy: http://www.michaelsmachines.com/
He has the correct tools to remove the barrels on swiss sig rifles. I would assume sig 556 would be the same.
He was able to cut the fh off behind the slots and then turn down the remaining portion. It left just enough meat to thread the barrel on my Sig 553 LB w/ Nato lower. It looks like it was made that way with my aac mount on it. It probably lowered the value of my rifle but I don't care.
I found him after much research finding someone mentioning him in a forum from a thread like a decade ago. I was lucky to find this info.