I must alologize about for the following text being so lengthy, so...
I think that your booster is likely stuck inside/stuck to the barrel jacket by way of fouling and not by pressure fitting.
Paul Harvey/Garrison Keilor Version
In researching the British ANM2, I was made aware of MK evolutions and changes made to these .303 Machine Guns in order to alleviate issues that cropped up.
One of the earliest issues the British encountered with this sized-down version of the USA Colt MG40 that needed to be figured out was how to keep the guns cool while firing in their primary location, fixed within the wings of RAF fighter planes. The British were already deeply into the manufacture of the .303 caliber cartridge, it having seen widespread usage by the English in the SMLE Enfield rifle in the muddy trench Hell that was WWI. The British became very familiar with the .303 and as the wings of their new fighters were thin, a light-to-medium caliber cartridge was necessary for space saving reasons to be able to fit the ANM2 inside a wing. Add that to the fact Britain already had the manufacturing infrastructure capability for the never-before-seen mass production requirements of the caliber well established established.
Incidentally, the ANM2 had to be installed on its side as positioning it prone would make the receiver and feed portion of the gun too tall to fit within the wing. This orientation presented other design criteria for the gun that set it apart from both the .30 version of the US ANM2 as well as the US .50 Browning ANM2 Heavy Machine Gun used by the Americans in both Ball Turret and Flexible Mount positions in US Bombers: both calibers of US ANM2 were oriented upright, where the the mount-on-side requirement of the British ANM2 carried the additional requirement that the Medium Machine Gun be capable of being belt fed its ordnance from either side. The .303 ANM2 also fired from an open bolt position, thought to help with cooling the 4 wing mounted guns by having the bolt and action left agape when not activated; both calibers of the US ANM2 fired from a closed bolt position, which had the benefit of alleviating any possible ammunition “cook off” events. This open bolt configuration is what gave the British ANM2 its legendary high rate of fire and helped make this Machine Gun “the Gun that Saved Britain” in the Battle of the Bulge.
Consequently, the extremely high rate of fire (as well as a few “minor” tweaks to the muzzle end devices) would cause problems for the ANM2 initially. Sending that much steel down the pipe so quickly caused intense build up of fouling (combustion gases plus unburnt powder plus hot temperatures) in the small bore of the original MKI booster causing catastrophic damage to both the boosters, barrel jackets, barrels, flash cones, and even occasionally the receiver and wing.
Several variations were tried out in an attempt to fix this often disasterous chain of events including: reaming the throat of the boosters (and flash cones), replacing the smooth flash cones (which had no MK designation yet I believe) with stunted compensator cones (or muzzle nuts, which is the MKI designation if I recall correctly), progressing back to a single piece combined muzzle nut/flash cone (now with heatsink rings (the MKII), and also changing the one piece flash cone/muzzle nut with the two piece combo we most often find now (designated MKII*). There are more variations than that including an upgrade which plated the inside of the booster with chromium.
Other changes to the rest of the gun included a barrel jacket with smaller air vent ports to larger vent ports as well as increasing the number of vent ports.
By the time the MKII** was moving into heavy production, the European theater of conflict of WWII was all but ended, with the British ANM2 having accrued a total build tally of 440,000 units with another 100,000 units worth of spare parts, manufactured by three factories on two continents (not including the original modified MG40 prototype guns built by Colt) seeing action and usage from Australia all the way around the world (the long way) to the Pacific.
If I had money to bet (I don’t), I’d say there is mass fouling built up in between the threads of your booster and the threads of your barrel jacket that has had over 70 years to set up shop and get comfy. You might say they Fused (like your screenname, LOL! Okay I’m a grandpa now so I’m allowed to tell that kind of dumb joke).
I’d guess a little heat will get that booster unwound. See newmagrathea ’s removal video in his thread or in The Ninja ’s collection thread for lesther strap usage to not damage the booster vanes.
*Fun fact about the extremely high rate of fire: the vibrations generated by the .303 caliber ANM2 came at such a fast pace, the intesity often times walked the flash cone/muzzle nut out of the booster (MKII) or the flash cone out of the muzzle nut (MKII*) or most problematic of all, the booster itself out of the barrel jacket, THUS, the welding nub on the booster which tack welded the booster to the barrel jacket.
Wow I learned something