I have a 444 Win 94 that i don't use, so I thought I would cook up a more useful combination for my purposes. In the interest of science I scrounged up a Rem SS 7mm RM barrel and cut it off and threaded it for the Big Bore Winchester. The goal was to have a 2.04 " case length which would enable me to use FN bullets designed for the 7-30 Waters and crimp in the cannelure. The case is the one with the 120 Sierra stuck in the mouth.
This lineup includes the 7-30 Waters, 7mm-08 Rem, 7mm Mauser, my shortened 7mag and the 284 Winchester arranged in order of increasing case capacity. The short 7mag lies midway between the 7 Mauser and the 284. Based on the info in a case forming handbook belonging to a friend the capacity is about the same as the 7mm Mauser AI.
This is one nifty little cartridge. My initial load testing is with 120 Sierras. 53 gr. WC852 gave 2660 fps. 53 gr. H450 gave 2909 fps. 51 gr. H4350 gave 2855 fps. 50 gr. W760 gave 2918 fps. 42 gr. Varget yielded 2749 fps. and 45 gr. IMR 7383 delivered 2599 fps. Primers are round with only a hint of a flat center in the higher velocity loads. E.S.'s are low and accuracy is fair. Groups ran between 1" and 2" and increased with velocity.
The only fly in the ointment is the platform I built this cartridge on. It really takes a good bit of pressure to open the action after firing one of the higher velocity loads. I am getting to much bolt thrust with the large base diameter.
Next step is to polish the bearing surfaces on the locking bolt to see if it will make it easier to open the action. Then I am going to work up a little more with WC852, IMR7383 and try some H4831. After that I will start playing with cast bullets. I have the Lyman 287346 and the Saeco 071. The Lyman may work better since the 7mag has a throat and the Lyman has a longer bearing surface. This case, with the reduced capacity, ought to be a little more cooperative than the full size 7mag.
Case forming is laborious. The easiest to work with is 6.5 RM brass. Base diameter runs from .505" to .506" in the batch I bought-just about perfect. I turn the belt and rim off with a lathe and then bump the shoulder back about 1/8". I am using a cutoff 338 WM die to bump the shoulder and then a shortened 7 mag die. This process has to be done in steps with a relube between steps to prevent wrinkles in the shoulder.
I can tinker with this one for a long time!