I want to build my first custom rifle. I just ordered my barrel and now I need an action. I'm building a 257 STW and just want to make sure that any Rem. 700 magnum long action will work or if I'll have to get a RUM cartridge to have a long enough action and magazine box. The magazine box will have to be at least 3.7 inches. Thanks in advance.

Yes, the Rem 700 long action magnum will be fine.

That's what I thought. I was hoping I could find a little better deal on a rifle in a less popular cartridge vs. a more popular round. Thanks.