Parker Otto (P.O.) Ackley was a well known author and gunsmith who improved standard metallic cartridges by decreasing the body taper and increasing the shoulder angle. These changes provided increased case capacity and increased ballistic performance. Ackley Improved cartridges could be chambered in a rifle in a preexisting caliber. The brass for the improved version of the cartridge could be fire-formed from a parent cartridge or formed with a fast burning powder and an inert case filler. Cartridges improved by Ackley, are known as Ackley Improved (sometimes designated by only the word Ackley or Improved). For instance, an improved version of the 280 Remington would be known as a 280 Remington Ackley Improved or 280 AI. Shooters might refer to it as a 280 Ackley or a 280 Improved.
Originally, the no-go gauge for the parent cartridge would be used as the go gauge for the improved version. The field gauge of the parent cartridge would be used as the no go for the improved cartridge. This would provide a safe chamber dimension for the fire forming the parent cartridge brass in the improved chamber, while requiring minimum work to improve a rifle barrel. The downside to this method was that firing the parent cartridges in the improved chamber would often provide inaccurate results and the parent case would stretch to the longer length when the case obturated against the chamber wall.
Some smiths turned back the barrel tenon one thread, reset the shoulder, and chambered the barrel with a headspace dimension .004″ short of minimum so that the parent cartridge would be supported at the neck shoulder junction and provide improved accuracy and less case stretching when fire forming. Resetting the barrel one full turn also allowed the existing markings on the rifle’s barrel to remain in the same spot when assembled. The only real downside to this method is that it requires a lathe, while the original method of improving a chamber can be accomplished by hand.
To properly headspace barrels chambered in this method, Dave Manson, of Manson Reamers makes specialized Ackley Improved go gauges. When using one of his improved go gauges, the bolt should close on the Improved go gauge. The parent cartridge go gauge becomes the no-go for the Improved cartridge and the bolt should not close on it. Keep in mind, depending on the headspace gauges used, the headspace can be .008″ shorter for barrels chambered with the set back shoulder method. This provided less case stretching and greater accuracy when firing cartridges loaded in the parent calibers brass.
The 24″ Bartlein Heavy Varmint Contour barrel I am “improving” in this article has already been chambered in 223 Remington. I will be re-chambering it for 223 Remington Ackley Improved.
The 223 Remington Ackley Improved is one of the most popular of Ackley’s creations. Parent brass is readily available and reported velocity gains average 100-140 FPS depending on bullet weight. 6mmBR.com has an excellent page with information on the 223 Remington Ackley Improved here.
I ordered the following supplies from Brownells (1-800-741-0015) for this project:
- Manson 223 Remington Improved finish reamer (513-050-223)
- Bartlein HV 1:8 twist barrel blank (749-008-379)
- 3/8″ high-speed steel turning took set (080-000-835)
- Viper’s Venom cutting oil (100-013-368)
I ordered the following from Manson Reamers (1-810-953-0732):
I also had Manson grind the fixed pilot reamer I ordered from Brownells to take interchangeable pilots. By ordering an in-stock reamer from Brownells and sending it to Manson to be ground, you can significantly reduce waiting times over having a reamer custom ground.
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All lathe work was conducted on a Grizzly 4003G lathe.
The rifle has a BlackHeart International action, which is a Remington 700 clone with 1 1/16″ x 16 barrel threads. To determine how much I needed to cut off to allow for a complete turn, I divide one inch by the number of threads per inch. In this case, 1″/16= .0625″. Setting back the shoulder .0625″ will allow me to chamber the barrel and have the existing barrel markings index back to where they started.
Properly aligning the barrel in the lathe is critical. If you end up cutting a new shoulder that is not perpendicular with the threads and tenon, the action and lug will have a space on one side and not the other. This will mean you need to completely recut, and thread the tenon and lose valuable barrel length.
I use a number of different methods to align barrels. Sometimes I dial in the last two inches of the chamber end, sometimes, like on this barrel, I dial in both ends. In this case, realigning the barrel in the lathe was fairly straight forward.
The chamber’s cut, its time to re-stamp the caliber markings, reassemble the rifle and head to the range to see how it shoots.