I used to say read the excellent instructions in:
this threadread report on page 3 of the same thread for tips and links to good wrenches but a lot of the pictures are gone, so it may not be as useful these days.
"The short version is torch the thing until it starts smoking (this took a LOT longer than I thought it should have) and keep torching it until it STOPS smoking (this also took a LOT longer than I expected). Then you can probably wrench it apart in a barrel vise with two of those Stahlwille wrenches. I tried the "step on the wrenches" technique but only after torching it the first time (didn't start smoking) so probably not a fair test - or I'm just a lightweight On this barrel, the flats of the check nut and flash hider were only 10-15 degrees apart, so also probably not an ideal case for the two-wrenches-and-a-fat-guy method either. YMMV of course, but use the blue wrench LIBERALLY until you get smoke coming off and don't quit until she stops smoking."