I've taken to test-fitting the components before assembly to avoid this problem, but have had to deal with it. I put a little thin CA glue on the inside of the upper barrel tube, and twist it so that the glue spreads around before curing. It may take a couple of applications, but the glue effectively reduces the inside diameter of the tube - resulting in a tighter fit.
It's also possible to deform the transmssion a little (by squeezing or dimpling) to snug it up.
Regards,
Eric