I know you said in your original guidelines nothing over .30. But then you let someone talk you into considering the .35 Whelen. So here goes....

You want something truly different, (maybe even esoteric....mysterious even)! Something that you will have no trouble getting brass or bullets. You want something to do things for you that your current battery does not do. And in keeping with the suggestion above.....maybe even "worthy...suitable" of the classic Enfield action.

Here are two "different" choices. Yes...they are both wildcats! Yes.....you will have to reload for either, (and need dies.....which RCBS can provide). But......you won't find either of them just sitting around in any old camp. And they both have the head stamp size of the standard .30-06.

The .375 Whelen Improved.

The .400 Whelen.

The .375 Whelen Improved can be (and most commonly is) loaded using .30-06 cases (actually .35 Whelen cases would be even easier) necked up and the shoulder angle steepened! Respectable velocities can be achieved.....and the cartridge very much resembles the "old English and Continental" medium bore cartridges very much in use in Africa around the turn of the 20th Century.

The .400 Whelen is best reamed and bored to use standard bullets as the .416 Rigby or the .416 Remington. So bullets are common enough. However, it is most common today to ream the chamber to actually handle the case commonly used in the .240 Weatherby. That case is essentially no more......and no less......than a .30-06 with a belt. Because the .400 Whelen has far too insignificant shoulder for positive headspacing.....the belt insures proper headspacing. I know the .240 Weatherby cases are a little less common.....and a little more expensive....but you end up with a truly different rifle. And in a very uncommon caliber. The .400 Whelen out performs as it is a true over achiever! You'd be surprised what a .30-06 sized case can do for a forty caliber bullet. It is not a "flat-as-a-string" trajectory.....but it delivers truly impressive performance over normal hunting ranges.

By the way.....a good source of info on either of these or many other cartridges is a book entitled, "Big Bore Rifles".

Good luck with your decision!

Dave

P.S. One other suggestion. (As long as everything is up for grabs.) An Enfield chambered in either of those and equipped with a good set of adjustable aperature sights.....would be as near perfect an outfit I could imagine for a grand old Enfield! Just my .02 cents.