I use barrel adapters quite a bit to run my older Spyder-threaded Kingman barrel kit on my AutoCocker-threaded markers, specifically AutoCockers.
They work really well, they'll add about 1.25" to your existing barrel but other than that you won't notice they're there. Accuracy hasn't suffered at all when I've got adapters on, and I'd dare to say it's improved on my pump.
Basically how they work is it's a tube with male threads of one type like a normal barrel, then a sleeve with female threads of another type, like the inside of your marker. You just screw the male threads of the adapter into your gun, then screw the off-threaded barrel into your adapter. Piece of cake.
Get a Lapco adapter if you do, though. The RAP4 A5-to-AutoCocker is a piece of crap. My Lapco is still going strong and is nice and tight.
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