Just an FYI
In gunsmithing school we had a 'barrel holding jig' type of device. It was a piece of milled steel that LOOSELY fit You put that around the barrel, fill it with molten lead. Let the lead cool. You now have a captive barrel that you can mount in a vice without marring the barrel
When you are DONE working on the barrel - ie you have removed the receiver or in your case, the flash hider - then you crack the outer steel mold off and then the hardened lead will come right off without harming the barrel. I can not say how it effects nitride finish though. I always worked on blued weapons.
Looks kinda like this type from Brownells...