Chambering a rifle for an Ackley Improved Cartridge

Rechambering a custom rifle for the 223 Ackley Improved

January 22, 2014 Bill Marr Gunsmithing 0

Fire-formed 223 Ackley Improved case (left) and fired 223 Remington case (right). The improved case (left) doesn't have a tapered body and has its shoulder moved forward at a sharper, 40 degree (versus 20 degree) angle.

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:

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.

The barrel is set up in the lathe. I use a .0001" indicator to ensure that the barrel is aligned. Failure to properly align the barrel with be problematic.
The barrel is set up in the lathe. I use a .0001″ indicator to ensure that the barrel is aligned. Failure to properly align the barrel would be problematic.

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.

When I originally chambered this barrel, I dialed in both ends, as opposed to just the chamber end, so I use a tapered range rod to make sure the barrel was aligned in the same manner as it was the first time I cut the chamber.
When I originally chambered this barrel, I dialed in both ends, as opposed to just the chamber end.  I use a tapered range rod to make sure the barrel is aligned in the same manner as it was the first time I cut the chamber.

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 first cut I make on the barrel is to shorten the tenon one full turn. In this case .0625".
The first cut I make on the barrel is to shorten the tenon one full turn. In this case .0625″.
Next, I recut the shoulder to the proper length. This is critical to get right, too short a shoulder will yield and unsafe rifle.
Next, I recut the shoulder to the proper length. This is critical to get right; too short a shoulder will yield an unsafe rifle.  Note the slight groove on the tenon, this was the undercut against the shoulder of the original tenon.
I then widen the relief cut behind the threads so the receiver can snug up against the recoil lug. Here I am test fitting the recoil lug.
I then widen the relief cut behind the threads so the receiver can snug up against the recoil lug. Here I am test fitting the recoil lug.
I screw on the receiver and lug to make sure everything is fit and aligned. I am checking two things here; first, I am making sure the receiver can be tightened against the lug. Second, I am making sure the lug is square against the tenon shoulder- if the barrel was not correctly aligned, there would be space on one side. Everything looks great here.
I screw on the receiver and lug to make sure everything is fit and aligned. I am checking two things here; first, I am making sure the receiver can be tightened against the lug. Second, I am making sure the lug is square against the tenon shoulder- if the barrel was not correctly aligned, there would be space on one side. Everything looks great here.
Since the tenon's shoulder was set back, the bolt nose recess is now too shallow.
Since the tenon’s shoulder was set back, the bolt nose recess is now too shallow.
I use a high-speed steel insert boring bar to cut the bolt nose recess to the proper depth. You can't use a piloted counter bore in this situation because the chamber has already been cut, the pilot has nothing to follow.
I use a high-speed steel insert boring bar to cut the bolt nose recess to the proper depth. You can’t use a piloted counter bore in this situation because the chamber has already been cut, the pilot has nothing to follow.
I check to make sure the bolt fits in place.
I check to make sure the bolt fits in place.
223 Improved go gauge (left), 223 Remington go gauge (center), and 223 Remington no go gauge (right). The 223 Remington go gauge will be used as a no go gauge for the improved cartridge.
223 Improved go gauge (left), 223 Remington go gauge (center), and 223 Remington no-go gauge (right). The 223 Remington go gauge will be used as a no-go gauge for the improved cartridge.
I set up a Mason piloted 223 Improved reamer in a floating reamer holder with an adjustable reamer stop.
I set up a Mason piloted 223 Improved reamer in a floating reamer holder with an adjustable reamer stop.  I selected a floating holder to allow the reamer to follow the existing chamber.
When the reamer stop reaches full depth, I check my progress.
When the reamer stop reaches full depth, I check my progress.
With the go gauge in the chamber, I use feeler gauges to determine how much deeper I need to cut the chamber.
With the go gauge in the chamber, I use feeler gauges to determine how much deeper I need to cut the chamber.
When the bolt closes on the 223 Improved go gauge, and doesn't close on the 223 Remington go (shown here), the chamber is the appropriate depth.
When the bolt closes on the 223 Improved go gauge, and doesn’t close on the 223 Remington go (shown here), the chamber is the appropriate depth.
Here is a piece of unfired brass that that had its neck coated with marker. Note the shiny area at the case neck and shoulder junction that holds the non fire formed cartridge.
Here is a piece of unfired brass that that has had its neck coated with marker. Note the shiny area at the case neck and shoulder junction that holds the non fire formed cartridge.

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.

Here is a Winchester 5.56mm 62 OTM cartridge (left), next to the fire formed brass (right).
Here is a Winchester 5.56mm 62 OTM cartridge (left), next to the fire formed brass (right).
Fire-formed 223 Ackley Improved case (left) and fired 223 Remington case (right). The improved case (left) doesn't have a tapered body and has its shoulder moved forward at a sharper, 40 degree (versus 20 degree) angle.
Fire-formed 223 Ackley Improved case (left) and fired 223 Remington case (right). The improved case (left) doesn’t have a tapered body and has its shoulder moved forward at a sharper, 40 degree (versus 20 degree) angle.
I would say this rifle shoots well even when fire forming. This was my best 5-shot group of the day. 69 SMK on top of Reloader 15 in a Lapua case.
I would say this rifle shoots well even when fire forming. This was my best 5-shot group of the day. 69 SMK on top of Reloader 15 in a 223 Remington Lapua case.

 

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