I'm gathering parts for a winter project. I am starting with a stripped No 1 Mk 1 action and have not yet decided what form of LE or LEC I'm going to part it together as. It's marked as a full length rifle, so I'll likely build it that way, however the prospect of assembling an elusive cavalry carbine with new wood and metal from Ross Restoration is very tempting.

The only component that concerns me is the barrel. I have not yet contacted any barrel maker, but I am frequently near the Criterion office, and have heard that Armalon and Shaw both make similar.
I may make some enemies by saying this, but I'd also like to stay away from 303 Britishicon on this one. 303 ammunition and components have been difficult to find recently, and I don't think that is likely going to improve. I'm trying to think 30 years in the future and figure out what I can still shoot then. At least in store, a lot of surplus and vintage calibers have vanished following covid. I have not seen a box on store shelves of US made 8mm Mauser, 303 British, 30-40, 7.63 Mauser, 38 Colt etc. in several years. I briefly thought that 30-30 would be gone as well. Reloading components are out there, but I don't know for how long.

While 303 is the obvious caliber for a Lee Enfield, I may consider other .30 cal range modern non wildcat cartridges.
The best all around I could come up with that would be safe for the action was 7.62x39 or the similar 300 blk, low pressure and powerful enough for a carbine, however I would need a custom extractor and magazine.
Another option I had was 7.62 Nato. The extractor and magazine can come from the Ishapore 2a, reducing cost. I already load light for Ishapore 2a's, and to my knowledge they reverted back to the original steel in ~1965, theoretically making this proposed rifle no less safe than a 2a1, ignoring cumulative stress from being significantly older and more used. This will lead to an unsafe situation should someone try to fire this rifle with high pressure 308.
Trying to think for the future considers cartridges like the new US 6.8x51, which could be assembled with the parts above, but has a maximum pressure much higher than its parent, which rules it out. 6.5 Creedmoor also has a high maximum pressure.
Other suggestions are appreciated.

This is going to be a unique project, and I don't know what the best path to take would be. I have done every part of this project, but not all to the same rifle, so I'll be learning the whole time. If this idea gets anywhere I'll keep updating.