I converted a Beretta Mato 338 win mag to a 338 RUM. However is has a long magazine at 3.685"
Here are pics of the 2 cases
So the case is .26" longer to start with. If your magazine is only 3.4" long like some of the win mag magazines are then you will not be able to rechamber to the 338 RUM and shoot any bullet with a pointed nose.
In my magazine I still have to seat the bullets almost to the ogive in order to fit my magazine at 3.685"
For instance here is a pic of a 225 gr TTSX and notice how it is seated all the way to the last cannelure