This is probably a question for fiftydriver, but I'll take advice from anyone in the know. I've got a win 70 338WM that I'm going to get rebarreled to 300 Wby (factory takeoff barrel). The internal magazine is already on the short side for the 338. It looks like a different box and follower could be put in to accomodate the longer round. I think that winchester has only one length of long action, so the bolt throw should be long enough. By the way, this is a controlled feed if it matters. Thanks! Sam