The action length is the same. The 300win Mag bolt face is larger and the feed rails are set up for the magnum cartridge. Basicly the same action. The Ultra Mag actions are also the same length. Again the feed rails are wider than the Mag rails. All are made from the basic (30-06) action with machining mods for the larger cases.
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where you can find links to LRH discussions about these products.