When I Bubba'd my '57 I paid a lot of attention to the barrel found that the end of the .22 version was counterbored by about 5.8mm by about 10mm deep You can't fit a .22LR very far in the counterbore but I guess you might be able to put a CB cap in there. I have a suspicion that they may have used low power firearm barrels rather than airgun barrels, but fitted backwards. The counterbore really won't hep accuracy. Mine is pretty accurate with standard JSBs after being chopped by 4", recrowned and retuned. Doing this so I could fit a silencer was my lockdown project (a lot of work) but just taking 2cm off the cocking lever and barrel is a lot easier.

The .177 version doesn't have this but it still has a short 6.4mm wide section that the barrel exits into. It might be improved by taking off the foresight and drilling it out to 9mm so the air blasting out of the barrel is not redirected towards the pellet.

It is worth making sure there is nothing under the loading port. Any dirt, fragment of lead or bit of seal will misalign it.

BB