This might be old news but it is new to me....I was about to cut off the ribs in a 338 Lapua mag to be able to use it in a 300 RUM build I am starting on, when I discovered that Accurate Mag is now offering mags in 3.715" and 3.950" lengths that are built specifically for the RUM cases and the 375 H&H. I am pretty happy I don't have to butcher these 338 LM mags.

http://store.accurate-mag.com/sssf30...75HandH-1.aspx

ITEM CARTRIDGE CAP W L H
00100-0045 300 RUM/ .375 H&H SSSF
3RD		 .736 3.715 2.95
00100-0047 300 RUM/ .375 H&H SSSF
3RD		 .736 3.850 2.95
00100-
0044		 300 RUM/ .375 H&H SSSF
5RD		 .736 3.715 3.66
00100-0043 300 RUM/ .375 H&H SSSF
5RD		 .736 3.850 3.66