The final fireformed .257 USM wildcat case starts as a .270 WSM factory case and is then annealed, resized, trimmed, and neck-turned.

Sometimes I get involved in a shooting project that can't be considered to be remotely practical. It's happened again. I've been working up loads for my latest wildcat rifle cartridge, the .257 USM. Yes, as the moniker suggests, it's .25 caliber, but no, the "U" is not an abbreviation for "ultimate." USM actually stands for "unimproved short magnum."

The idea for this wildcat started when Winchester and Browning invited several writers on a caribou hunt in northern Alaska about three years ago. The bush pilot ferried us about 30 miles north of Kotzebue to a base camp situated along the Squirrel River in the Baird Mountains.

We couldn't hunt the same day we flew into camp, so we were just settling in and getting acquainted with each other. The conversation veered to what the next WSM and WSSM cartridges would be. A veteran hunter/writer asked, "What happened to the .257 WSM?"

The company rep replied that although it had been on the to-do list, the higher pay grades declared the proposed round as ballistically untenable and decreed the .25 WSSM as a better option. Our host was so disturbed by the decision that he went ahead and ordered a custom rifle chambered for a necked-down .270 WSM.

I later purchased a couple Model 70s in .25 WSSM after deciding it was the best of the three super-short magnums. Performance mirrors the .25-06, but the compact WSSM package is much handier. I also have a Weatherby Vanguard Sub-MOA chambered in .257 Weatherby Magnum. Cartridge efficiency is definitely an oxymoron for this round because, with the heavier bullets, you have to load half again more propellant compared to the .25 WSSM to achieve, at most, 15 percent greater velocity.

A couple years later, I read a glowing review of the wildcat .25 Pronghorn, the necked-down .270 WSM round we'd discussed on that earlier caribou hunt. It sounded like an interesting project; however, the more I considered it, the more I thought that integrating the two Winchester factory rounds might even be a better idea. So overlaying the neck and shoulder of the .25 WSSM on the full-size .270 WSM case formed the .257 USM.

I essentially just un-improved the already overbore-capacity round.

Surely, wildcatting a wildcat can't be practical.

To make matters even more impractical, I decided to build a rifle from scratch instead of simply rebarreling a factory model already chambered for one of the short-magnum cartridges.

I had a reamer cut that altered the front end of the .270 WSM chamber dimensions to those of the .25 WSSM. The shoulder starts about fifty thousandths of an inch farther back. So the neck is a bit longer, and the shoulder angle is also less abrupt--30 instead of 35 degrees.

Next, I chose a Montana Rifle Co. Model 1999 medium-length (3.100 inches) action instead of short or standard length so that I could seat the heaviest .25-caliber bullets no deeper than to the base of the case neck. Gunsmith John Gallagher blued the action; fitted a stainless, 26-inch, Lilja barrel (three-groove, 1:8-inch twist); and bedded the barreled action in a take-off laminated Winchester Model 70 Coyote stock.

RCBS supplied a custom set of Gold Medal bushing dies to complement my custom chamber specifications. I hoped to avoid annealing the parent .270 WSM brass, so I reformed a few cases and made up dummy rounds for Gallagher to use to headspace the barrel. Several months later when he completed the project, I noticed one of the dummy rounds exhibited stress-corrosion cracks. I definitely had to anneal the cases before reforming them.

Successful annealing is shown by melted indicating fluid and uniform bluish discoloration of the case necks and shoulders.