I'd have to disagree. The 300 Win Mag has a case length of 2.62" while the 300 PRC is 2.58". The 300 RUM has a case length of 2.85" which would require modifications to the magazine box.
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You can't go by just the case length. You have to look at the overall length of a loaded round and the PRC is longer at 3.7" according to Hornady vs around 3.34" for the win mag. So yes the case is shorter but if loaded according to Hornady's spec's, it's not gonna fit. All the articles I've read on it say it takes a full length magnum magazine.