I actually increased you case capacity by 4 grains. Per your post your imp. version is only giving 3.5 grains over a standard 375 RUM. You're throating it so you can seat the bullet out to 4.200 OAL , Ok, great but that just gives you more useable case capacity because you aren't taking it up with a deeply seated bullet, also taken into consideration. So unless you have some hybrid designed throat, nothing else is different and you are wellllllllllllllllllll over max pressure which is why you are getting crazy fast FPS and only 2 go rounds on your brass. I'm guessing the primers go in pretty easy after 1 firing and you probably can't keep them from falling out after 2.
Remember, pressure shows in a variety of ways. You probably aren't seeing any heavy bolt lift because you have a case that is nearly straight with little taper and improved type cases hide pressure signs like heavy bolt lift. You're producing FPS numbers a 378Wby. can't match and it has a MUCH larger case capacity. Your chronograph is telling you to back off but maybe you aren't listening
Rem. brass certainly isn't Lapua quality I'll give you that but it last longer than 2 firings. Unless you're way over pressure.
Feel free to shoot it however you choose, it's your life.....
