The standard AI mags are identical in length for teh 300WM and the 338LM magazines. The only difference that I can tell (I have and use both) between the two are the length of the feed lips.
Some manufacturers do make a "CIP" length magazine, and that gives you a slight bit more useful length, 3.750" versus 3.650." I do not know that standard AI magazine bottom metal can use the CIP magazines.
Accurate Magazine makes one that goes 3.850", but I'm thinking that is an external dimension, so I do not know what it gives you from maximum interal length: