I want to build a rifle with interchangable barrels. The rifle won't be designed for changing barrels in the field, I intend to use abarrel vice and a wrench that locks through the action. I have seen a few rifles done like this and they work fine. The question is what other cartridge will feed nicely from the .375HH magazine and use the same boltface? I though the .450 ackly might be the go?
Interested in .416 upwards???

Any input on calibre / chambering would be greatly appreciated.

You would be smart to go up to the 416 Rem. and/or down to the 300 H&H, they both would feed and extract properly because you made no change in the case to speak of...A three shooter that sure covers the whole spectrum....wow!

No doubt about it, a 375 H&H and a 300 H&H would be as cool as it could ever get.

Thanks I already have a .300 so don't need the double up (while I have other calibres I need).
The .416 sounds like the go. Thanks.

any comments on the Ackley? Not suitable? I thought there was another 450 cal that was the .375h&h shortened and necked up????

470 Capstick
A .475" 500gr @ 2400fps, 6400fpme.
You may need to modify your stock/bedding to deal with it, though.
If it were me, it'd be the 416 Rem.
There's the shorter cartridges, too, like the 458 WM, 425 Express, 416 Taylor, which are all essentially the blown-out H&H case shortened to 2.5" and should feed fine, right?

The other .450 cartridge, that you might be thinking of, is the .458 Lott which is basically a .375 H&H blown straight out to .458 without a shoulder. Alternatively you could look at it as a stretched .458WinMag.

I would opt for the .458 Lott over the .416 Remington Magnum to give you more big-critter-performance over the .375 than the .416 would provide. However, you are certain to have less problems getting the .416 Rem to feed out of a .375 magazine, although the .458 Lott might also feed without problems - the only way to find out for sure is to try it but that could be an expensive experiment if it doesn't work.

If you decide on the .416 Remington Magnum, I have a brand new Clymer finishing reamer for sale in that calibre - if your gunsmith doesn't have one.

It was the watts I was thinking of. Immediately, I might go to the .416 taylor on my existing .300 win mag and see if the logistics work. If it all goes OK, I'll grab a .375 and go as big as I can.