The New 25KPC Cartridge – Testing A ‘Quarter Bore’ AR-15

I had my first experience with the .25 caliber ammunition while filling a pronghorn doe tag in Wyoming. I was shooting with a borrowed Mauser that had been re-chambered in 257 Roberts. That was a great rifle, accurate, easy shooting, and it did a fine job on the pronghorn.

I’ve been smitten with the ‘quarter bore’ round ever since. I’ve always been a bit surprised, given every other caliber used, that the .257 bullet hasn’t been as explored for use in the AR-15 platform. That appears to be no longer the case, as the 25KPC is nearing its debut.

My consistent friend and occasional gunsmith, John Stewart (the maker of my “Last Rifle”) has been developing the cartridge for a few years now. The goal is a reliable cartridge for auto-loading or lightweight bolt action rifles that’s most suitable for long range varmint work, but still capable of ethically taking deer at the common ranges at which most people hunt. Stewart shipped me an AR-15 upper with a 16″ barrel chambered in the new 25KPC. I’ve been putting it through its paces for the last couple of months and it’s performed admirably.

Stewart took a Manson reamer to the stainless-steel tube and placed it in an upper receiver and shipped it to me, along with some test ammunition and reloading dies. The maximum length of the cartridge is 2.310″ and .257″ bullets ranging from 75 to 100 grains are the best candidates for the cartridge when used in an AR-15 platform rifle.

With the longer cartridge and the overall length of AICS-style magazines, 117gr bullets are possible for use in lightweight bolt action rifles. Using the 30 Remington/6.8SPC as the parent case, fast burning powders, such as the Hodgdon’s CFE BLK work well in the new round.

At the time of this writing, no commercial brass exists for the cartridge. Brass from the .30 Remington/6.8SPC case is necked down and then fire formed. Given the parent case, the delta between the body and the neck isn’t as extreme as some other cases, making for reliable loading and cycling from the magazine.

With the supplied barrel and upper, 75gr VMax bullets over 26.8gr of CFE BLK produced an average velocity of 2,759fps, when measured with my Garmin Xero C1 Pro chronograph. Heavier 100gr bullets will reach 2,600fps. Given the load development sheets Mr. Stewart offered, neither of these would be max pressure loads.

With the supplied upper mounted to another rifle’s lower and placed in an Ultradyne Launch Pad shooting rest, four strings of five rounds of the 75gr V-Max-topped cases averaged .8MOA groups at 100 yards when using a 10X scope.

That’s really not accounting for the cartridge as there are too many variables that depend on the firearm itself. It might shoot a whole lot better or worse depending on the barrel, trigger and receiver set. That said, this particular setup proved very capable, at least on paper. Fortunately, it was early in deer season here in central Texas and I had plenty of use for the gun in the field.

That V-Max load would be ideal for coyotes and a wide range of varmints as well as the smaller white tail deer we have in the Texas hill country. On a recent MLD doe cull, I put the 25KPC to test with the 75gr V-Max bullets. At ranges within 170 yards, the little round hammered our small does. Broadside double lung shots at 150 yards resulted in quick kills, full passthroughs, and sizable exit wounds. One frontal shot taken at 100 yards struck the heart itself, practically obliterating the organ.

Like many other .25 caliber fans, I like the quarter bores for their light recoil and “just enough” punch for a wide range of applications. Further load development, especially with the 100gr or slightly heavier bullets needs to be done, but the 25KPC shows a lot of promise for folks looking for a versatile, practical autoloading cartridge.