I occasionally experience the same problem, although it is not strictly caliber-specific. Some tubes simply fit the transmission snugger than others. When I run into a loose one, I put a drop of thin CA glue in the upper tube and roll it around so that it coats the inside all the way around. After the glue cures, I try the fit again. A layer of thin CA reduces the inside diameter of the upper tube a tiny bit and helps tighten the fit. Sometimes it takes a second application, but I have had good success with this method (better than trying to dimple or deform the transmission).I have one nagging issue that I wondered if you experienced it. When I'm making the pen with a 30-06 shell, the upper half of the pen does not fit snug enough to grab the transmission to open and close the pen. Its OK when I use a 308, but the trans sticks out further. Any suggestions?