got back to a conventional computer. Here is an excerpt from another post that explains it much better than I could:
From:
http://www.remingtonsociety.com/foru...php?f=4&t=2983
The 7mm Mauser cartridge dates from 1892. When SAMMI standardized the cartridge in the 20th century they settled on a case length that was slightly shorter than the original 7mm cartridge that rolling blocks are chambered for. This results in a modern 7mm Mauser cartridge having excessive head space (being too short) in the rolling block chamber. The distance from the base of the case to the start of the shoulder is shorter. It is difficult to fire form rimless cases as there is nothing holding the base of the case against the breech block. A successful way to fire form 7mm cases for the rolling block is to 1st expand the case mouth of an unprimed case with a .30 cal expander (I use a .30-06 die). LUBE the inside of the neck. Next, using a 7mm Mauser sizer die, screw it down until it only slightly sizes the neck. Try to chamber the case. In small increments, repeat above until the case will chamber, the breech block will close AND the hammer can be lowered to where it would strike the primer. It is best if there is very slight resistance of the face of the hammer against the breech block. You have now removed all excessive head space. Lock your 7mm die at this location and repeat the process for all cases you plan on shooting thru this rifle. Clean cases and reload using the starting load listed in modern reloading manuals. After firing it will be necessary to trim your cases to uniform case mouths.
There are other examples on the internet with the same explanation. I have a couple of rifles where it is best to fire form and neck size. This is the only one that is rimless. Another challenge.