Wild Bill,
I have seated some 700 gr FMJBT Mil surplus bullets so the crimping groove is just ahead of the case mouth and the M8000 will eject these live rounds easily. I do not have a measurement as they are over in the shop currently.
The 750 gr A-Max will be significantly longer, probably by at least 0.500" and I am not sure if the M8000 will eject a live round nor feed these rounds. I will have to see where the OAL comes out to during load testing.
You could make the OAL much shorter by cutting the chamber with a very short throat but that would take up alot of case capacity with the large diameter bullet and relatively small diameter powder chamber.
I got some of Richard Graves short baring surface 750 gr ULD RBBT HPs to try. THey are a very short bullet designed for big game hunting with 50 cals and with the much shorter baring surface then a normal 50 cal if this weight, velocity should be much higher as well.
Also, the OAL of the round will be much shorter. BC is less though, no free lunches I supose!!
I would stay away from the shortened BMG necked down to 375, just more case capacity then you need for that bore. My 375 AM reamer should be here soon and in my opinion, that will be about the ultimate on the 408 case as far as velocity, energy and trajectory even compared to the 338 version. Only problem is available bullets, we need some heavies. In my opinion, Sierras proposed 375 gr 375 cal SMK is about 20 to 30 grains to light!!
Kirby Allen(50)