EDITED> Begin with tube slightly longer than spec... I cut with hacksaw.... glue into blank......then gently insert the barrel trimmer into the distorted end by hand... not under power. Will round out with reamer. Then proceed to mount and trim with barrel trimmer on lathe.Cut them long glue them in the blank then trim to length using your barrel trimmer or sander or how ever you like to do it.
If using a barrel trimer I never trim on the lathe I hold the blank with a clamp and use the drill press. I find the chuck on the lathe and a vise on the drill press can cause the blank to reference off the sides instead if the tube. It also causes an do stress in the blank as the barrel trimmer is trying to force the blank square with the tube. Holding with the clamp allows the tube to easily stay square with the barrel trimmer in turn reducing the stress on the blank. I find it produces a much better cut for me. Also I as well cut the tube with a hack saw. If you drill a hole in a block of wood that is approximately sized for the tube then cut through that hole with your hack saw you will have a jig to cut that size tube without as much deformation.I cut with hacksaw.... then gently insert the barrel trimmer into the distorted end by hand... not under power. Then proceed to mount and trim with barrel trimmer on lathe.