If the adjuster and barrel are both steel then they may be just forced together(you do not mention if the are NOS or used) . If the adjuster is alloy and the barrel is steel then electrolysis has taken place and they are corroded together ,in this case I would apply heat (remove rubber o ring) to the adjuster and then twist. Alloy should expand enough to be moved.
BOB FREITAS
blue skys in MILL VALLEY,
CA USA