I had one that was really dry once on an FAL. And they torqued the hell out of it. It just took a LOT of torque to get it off. Be careful you don't damage your barrel if it's that tight. If you've tried heat and you're sure they don't use any thread locker, then penetrating oil is about all you have to work with. That or rebarrel or cut and rethread if it's worth doing that.
Soaking in hot water, then leave overnight, then soak again in hot water while you get your tools together is a good way of removing one with Rocksett, just be sure you thoroughly clean the barrel and all after so you don't get rust down in the barrel from water accidentally falling in there.
If it's on there that tight then there could be a chance of it affecting accuracy. If it's a good barrel now, check it after to be sure. Generally these things don't get torqued on all that tight anymore and even when you do with a crush washer, the crush washer takes up a lot of the stress. I've taken to using red loctite recently and had much better luck with it regarding removals and overall performance. My problem is I can't remember what was done with what!