Thank you all for your input. I'll confess that what's driving this project is a desire to firmly establish the large end of things on my rifle rack, plus availability of Montana PH action, more than perceived need. I'll admit that I hunt deer with a .375 H & H Ruger #1, but have a lot of fun working out the loads. If I was limited to using a .30-30 meat rifle, I might lose interest in hunting all together.

For the Gibbs project, I have been thinking 21" would be about right, and do the rest with load development and different powders. John Taylor, in "African Rifles and Cartridges" comments that the .505 has "plenty of power for all emergencies: so much so, in fact, that it would comfortably stand a reduction in barrel length to around 22" (with factory ammo)."

Playing around in my youth with long range revolver shooting Elmer Keith style has convinced me that you don't need a 26" tube for accuracy, and with the right powder it should be able to deliver the goods at 100 yards with reasonable efficiency out of a shorter barrel.