I fitted a longer barrel from a 4.5mm Gamo CFX(memory loss) on my PP700 pcp pistol and a laminated stock to convert it to a "carbine".
I had to thread the barrel around 40mm from the crown and now the barrel screws a lot deeper into the silencer than the normal rifle. The Gamo barrel had the plastic front sight which I removed obviously. The "carbine" is a lot louder than with the standard 5.5mm standard pistol barrel. Sound like you are popping a plastic bag, even with JSB Heavies. Am thinking of pulling a sleeve over the front barrel to bring it up to standard size and then thread it again. I'm a bit scared of just cutting the barrel to where its a uniform diameter, dont want to lose the "choke" the standard rifle have.
I build rifle silencers from steel for hunting rifles on a weekly basis almost but ain't so sure about airrifle silencers as there is a lot less pressure and no flame involved.
Any advice or opinions?