I tried a design with a hop up that slipped on the end of a barrel. I did not try slotting the end although I wanted to I just could not machine it good enough. It would be easy to do if you only went in an inch. I tried to go in 4 inches. It would look like this, one slips over the barrel with a set screw and the other end has slots. The holes all around are just to reduce weight is all. For a springer you would need a big air cylinder I suspect.
Here is the one I did below. It had two 5mm holes countersunk 1/2 inch end that intercepted the barrel at 10 and 2 o-clock. It was like a mickey mouse head, Then I stuck in an o ring with a tapered head screw that squeezed the o-ring into the bb barrel area depending on how hard you tightened the screws. Not a fan of dual adjustments, it was finicky.
That was a year ago. The kids used it for all sorts of things and I can not seem to find the 3 originals. Once I started working on a whole gun I lost some interest in testing this and never fully tested it. wife and kids are all like "oh ya I remember seeing it" but no body know where.
But that basic design could be altered with just gas escape slots at the end like the CAD mock up without any mechanical touching of the BB which is what I did.
Here is the barrel end if the mickey mouse version I did.