Any short mag. I would expect, would likely work with the the 7mag magazines.

For me, the .338 is tops for that rifle & would likely work in the same magazine.

Is the Bar gas, delayed blow-back or recoil operated?

I personally see 7mm Rem. Mag. as being inferior to the standard .30/06 with 24" bl. I've had a couple of the 7mm's and didn't like them. One was a BAR back in 1972. I traded it back to the gun shop for something else and don't even remember what. lol