I've only made one cartridge pen, and it happened to be the 300 Weatherby Magnum. The casing is long, and in this case, I didn't press the bullet in far enough, making the pen even longer. It is a 7mm tube, seated against the bullet, and cut flush at the end. The tube was epoxied in place, not soldered. The transmission was pressed in farther than any other 7mm pens I've made, but the epoxy was flexible enough to allow the tube to expand properly and let the transmission still operate. Even with the bullet extended as far as it does, there is enough transmission sticking out for the cap to engage and allow it to operate.
This is the second cap made for this pen -- the first one was shorter, to make the whole pen about the same length as a regular 7mm slimline, but it was too short for the clip. This cap is longer only to make the clip fit proportionally, making the overall length of the pen is longer than average.
This was given to a co-worker, and has been his daily use pen now for several months without problem.