I originally posted this thread in the custom rifle section and got some helpful responses. I should have posted here to begin with.

I am interested in working up a 338 win mag and have both a 264 win mag and a 270 weatherby mag as potential donors.

I believe the 270 WM has a case length of 2.545" whereas the 338 is about 2.5". It would seem this would not require significant alteration, but the 264 win mag donor is pretty much a a 'drop-in'. I'm just not as certain that I want to re-purpose the 264.

My question is how much more 'smithing will the 270 WM require than the 264 to become a 338, considering both the action and the box?

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With some actions no problem at all. With others it would be completely impractical.

I have a few Model 70's that I converted into switch barrels, all were either 300WM or 338WM donor actions, they now have barrels ranging from 257 Weatherby, 270 Weatherby, 264WM, 300WM & 338WM. None of them needed ANY alterations for feeding, mag box or follower changes.
The Rem 700's I have done also, do not need any alterations going from one mag case to another.

Cheers.

By the time one buys a good barrel pays for the install on has over half the price of a good used one or more.

Buy another rifle in 338wm and be ahead.

Yes, a good gunsmith can rebarrel either of these rifles to 338Win but the cost just doesn't seem justifiable since there are factory rifles on the market that will shoot just as well as a custom. My S-I-L has a Ruger m77 in 338Win that has been glass bedded and a Timney trigger that is way more accurate beyond most shooters capability with hand loaded ammo. Just my 2 cents worth.

If you have a .264 win mag and a .270 weatherby, to me they are essentially the same performance on game, so making one into a .338 mag seems perfectly reasonable to me.

Either gun should be fine action wise with making it in to a .338. Since you have your doubts about repurposing the .264, I think the decision is made for you.

I agree that a .338 mag can be had for a lot less than custom work, but the length and case head diameters are essentially the same, so it could be as cheap as a rebarrel. It also matters if part of what you want is to upgrade the rifle otherwise...wood, etc.

But from a technical aspect, it is not a problem.

Winchester introduced the .458 in 1956, followed by the .338 in 1958 then the .264 in 1959. The .300 Win came along in 1963.

The .338 and .264 are derived from the .458 which would explain why it is an easy job to use your .264 as a basis for the .338 mag.

Maybe if Norma hadn't come along with the .308 Norma Mag in 1960, then just perhaps Winchester would have created the .300 Win Mag, by necking down the .338 ?