SpudFiles

take a 3'' male adapter( what i use to innerchange barrels luckly)
the inside diameter of the threaded part is very close to the outside diamter of 2.5'' sch 40 pipe.
i cut a 3'' pipe a little longer than my barrel and glued it in to the male adapter as normal.
then i primered and glued the 2.5'' barrel and slid it inside the 3'' pipe and male adapter and flushed it with the tip of the fitting.
i then used the plastic peices from roofing tacks and shimed the barrel between the 3'' pipe the matain strightness.
the barrel attaches to a 170 cu. in. chamber made of 4'' clear pvc and launched a tennis ball today around 200yrds. in a matter of a sec. or so
will post pic of barrel and gun tommorow after work.

I guess that would work okay ,I made one today but a little different. I just used a short piece of primered 2" pipe with a 2" male or female ( I made one of each) fitting attached. A 3" piece of 2" would be adequate. I put on a lot of glue and let it set a few hours and it works quite well . If ever needed ,but I don't think it will be , some threaded screws could be added near the fitting. The 2" is a close fit ,well close enough to work inside the 2.5" pipe. I used conduit as that is all I could find locally. It is working really good.