I recently took on a project to inlet my choate stock on my Savage 110 338 RUM for a Mag conversion. I purchased CDI Precision bottom metal and a 338 Lapua AI Mag. My RUM shoulder is a bit longer and hit the inside of the mag on the roll for the follower guide. I just cut out a few large spacers and a piece of 5/8" square key stock. I inserted the spacers and shims to hold the steel square stock inside the mag and used a hydraulic press to flatten out the inside edge of the roll to clear my Rum shoulders.
Now the mag DID deform from pressing. So if you do a mod like this, be aware you may have to straighten the mag body afterwards. My front and rear of the mag bowed out slightly even with the precautions I took to avoid this from happening.
After, I TIG welded a small 1/8" tall riser on the back side of the mag body to keep the head of the round from slipping out the back of the mag. My feeding issues were resolved. No mods to the lips was needed.
This project did take some work to get working properly, so keep that in mind. Of your able to order a mag already in your caliber, your set. AI and everybody else doesn't make a mag for 338RUM. That was my problem. 338 Edge might have the same issues I did, but not sure.
