.251 TCR is about to be listed on Ammoguide.com, and this prompted me to take a closer look at loaded case dimensions. The case diameter is actually .272, not .275; the chamber is reamed to .275 however. The case does not taper, as you can see in the cutaway.
A word about AmmoGuide- they’ve been around for a long time in internet terms, and it shows; their website is archaic. It can take some effort to navigate, and you need to make sure JAVA is active in your browser. That being said, it is a treasure-trove of cartridge and loading data, the like of which I have never seen on the internet. This is a must for anyone into reloading, and for folks shooting wildcats, antique or obscure cartridges it is a gold mine! Well worth the modest yearly subscription.
Of course you can add cartridges (like the .251 TCR.) if you don’t see them, and you can add load data of your own for the cartridges listed. I highly recommend giving it a look.
Michael Tinker Pearce, 31 December 2019