I just looked up the case trimmer affectionately known as "trinket"...........Having seen how it works= you might be able to get away with it but the 338 rum mandrel would be to short.. the 300 rum mandrel would be the right length but would not keep the neck centered, The case holder might be able to but it looks like it would be problematic. Extreme runout would probably be the end result. I expect a big magnum wildcat is a great justification to invest in a trimming lathe. If I were you I would pick one up. I have the hornady and the rcbs and prefer the rcbs but like I said earlier had to dremel a little so it could pull out far enough for the edge cartridge.