Hi d-a i think the largest projectile you can use in a BR case is .375 but the 358 is a more practicle size for the largest. You cant go to 50 cal as the base of the case is only .470 so even a 458 projectile will not fit. The larger sub sonic rounds are usualy based on larger cases like the 460WBY or 500 Jeferies. the 458 mag can be used to make up a 475 but that is the MAx for the standard belted case and the ultramag can not go over 458 Max.
I just have to get a good detachable mag for the BR case that is light enough for my purpose the main thing is that it will feed and extract reliably. I have heard that a Sako type extractor makes a large diference in the ejection of BR cases in the Rem Action. The only problem with using the 6.8 Case at its length is the projectiles may be to long to feed through a mag when loaded to their shortest length aand if i shorten the case to much it may not eject well.
Is their any one with a 221 fireball or 300 whisper how well do they eject? what about a 7mm/221 or 270/221 would it have enough capacity to launch a 200gr projectile in 7mm or 270 with the 195gr Wildcat?