I used to have a list of all the proprietary Allen cartridges that listed their parent case, max velocity and their intended application...but I can't seem to find it on my hard drive this evening. It's hard to keep up with Kirby and his love for bigger and faster. Now that he has finished tinkering with the Cheytac 408 case, I think he has decided to skip the 50 cal. case and move right on to the Howitzer!
If I am not mistaken (and this happens more, the further north of fifty I get), the 338 AM is based on the 408 Cheytac case - a much larger case than the 300 RUM, on which the edge is based. Perhaps a better comparison would be between the the edge and the 338 Allen Express, which is based on the 338 Lapua Magnum case. The 338 AE case has the sides blown out and the shoulder moved forward with a slightly different angle (if I am wrong here, I am sure Kirby will jump in and correct me). I tend to think of the 338 Allen Express as a 338 Lupua Magnum that has been ackley improved.
For most of us, the ballistic difference between the edge, the 338 LM and the 338 RUM are just not big enough to bother with. It may exist on paper for those who like to debate ballistics, but in the field they all work well.