I did a similar mod on the first go round with the AC build up. I only used one tube, slotted as you described. I didnt want a longer gun, so I wacked 2 inches off the barrel. Gave space for the internals. Epoxied it on.
Warning: properly applied, epoxy can be stronger than the solder joint. (which is how I got a donor pump tube)
I assume that the breach end of this assembly is butted up against the breach and made to fit flush?
Very cool, and creative. Still have off center/eccentric internals? Shouldnt be an issue (hehe for you anyway )? or you got something in mind to solve that little snag?
dr_subsonic's pneumatic research lab
the Lunatic Fringe of American Airgunning
Southwest Montana's headquarters for Airgunning Supremacy
Proud Sponsor of team_subsonic