I have taken several of these apart (when needed) during the manufacture of 2-pcs. Blank-Barrel-Assemblies.
Other things that you can try are a heavy-duty vise with good/curved clamps to prevent the vise's jaws from cutting the barrel's outside surface......like good "Barrel-Blocks".
Heating of the barrel-extension slightly sometimes assists (with a couple drops of penetrating-oil) ...... but often it is just a good strap-wrench (or even a large adjustable wrench) that will break the extension loose from the barrel. Once loose, they just spin off by hand.
In extream cases, I have used a brass-drift and BFh (large hammer) to smack the flats of the barrel-extension to assist in loosening it up.
US Army, RetiredBlanksguy2001@outlook.com