I just bought a 1917 Enfield sporter(edit) and it turns out it is a 338-378 KT. Production dies are available. Seller may be able to track down the dies that went with this rifle, which would be great.

I have a 338/378 KT. I have never heard of the 30/378 KT but I assume it was the same cartridge as the 338, just with a 30 caliber neck. If so, it is the 378 Weatherby case with the shoulder moved back 1/4". For the KT you need a set of forming dies to set the shoulder back and either a neck ream die or turning the case necks as they will be to thick without this. You also need the correct reloading dies.

The easiest option would be to get a chamber reamer for the 30/378 Weatherby and rechamber to that cartridge.

Another option would be to get 30/378 dies and shorten them by 1/4". You will still need to neck ream or neck turn. I also don't know if you would be able to set the shoulder back with standard dies.

Edit: .338, not .30

Assuming the cases are the same except for the neck diameter that should work. You might try to find some Saeco stubby dies in 30 caliber. You also might be able to do it with the Weatherby brand dies that have the interchangeable bullet seating insert and neck sizing insert. I have the 340 Weatherby dies and they will work for both my 338/378 KT and 338/378.

I have a friend that has a 338 Lapua AI. I have it in front of me with a 338/378 KT case. The overall length of the KT is considerably longer than the Lapua. The shoulder on the KT is also longer than the Lapua.

If it were mine I would rechamber to a 30/378. Making the KT brass, neck reaming and fire forming is a real pain. I have not done it on my 338/378 KT because it is a Champlin Haskins with a 2 digit serial number and I am not about to destroy the collector value.