Under normal conditions, the brass tube expands slightly when you press in the transmission. When glued or soldered into a brass shellcase, however, the tube has no place to go. The solid portion of the case head (called the web) is too rigid.
On a 30-06 shellcase, the web is only about 1/4" deep, and the transmission typically seats farther in than that. Therefore, if you ream out the tube with a 1/4" drill bit, 1/4" deep, the transmission will slip more easily past the restriction and press fit where the tube is uncontstrained.
I ream my cartridges with a letter "F" bit, which is actually a little bigger in diameter than 1/4". The transmission then drops freely through the case web.
The trick to drilling a concentric hole is to drill on the lathe.
Regards,
Eric