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Rechambering for the Ackley Improved 07/17/2011 - By Tim Whealton
It might be more work than you thought but doing a good job usually is!
Maybe you heard the term but are not exactly familiar with the Ackley conversion but it has been around over 50 years. A brain storm of the famous P.O. Ackley, it is a unique way to increase performance from many of our favorite calibers.
The Ackley Improved conversion is essentially an enlarged chamber that by clever design will still allow the parent cartridge to headspace and be held tight against the bolt face. Upon firing the brass will be blown out and forward to conforming the brass to the Ackley Improved dimensions.
The Ackley Improved case will have dimensions that are straighter in the body with less taper. This increases capacity which makes for more powder and increased velocity. The most notable feature of the improved case is an increase in the shoulder angle. Usually the new shoulder will be a 40 degree but not always. This sharper angle shoulder contributes to more consistent headspace and less case stretching. It also seems to improve barrel life by reducing throat erosion.
So if the new case is so much better then why aren't they made this way to start with? Simple, you can't form the sharp angles with the forming dies used to manufacture cases. The only way to form the brass is by fire forming. This means firing the rifle to make a piece of brass. It isn't complicated, you just load a factory round and pull the trigger. It even shoots fairly well with the factory load! Matter of fact this is another of the real benefits of the Ackley Improved. If you are out of ammo you can shoot the parent case. This can be a large thing if this is your favorite rifle and you are on the hunt of a lifetime but your ammo runs out or is lost. If you were using one of the other wildcat cartridges it would be all over but with the Ackley, no problem!
For most hunters and casual shooters forming brass is nothing more than a minor inconvenience but if the shooter happens to be a competition shooter that uses a lot of brass it might be another story. Consider a Highpower Rifle competitor that needs 300 pieces of brass. Now if this same shooter is using a 243 Winchester that has a barrel life of 1200 rounds that might mean giving up 1/4 of the barrel life to make brass. This isn't to say that shooting the parent cartridge wouldn't be accurate enough to shoot the 200 yard portion of the match but after these shooters drive 500 miles and pay entry fees to shoot they don't want to have any variables like a case that doesn't fit the chamber perfectly. Some are even hesitant to use new brass that hasn't been fired in that particular chamber before!
It seems the most asked question about the Ackley conversion will be how does it group when fire forming. I have tested several through the years and most did better than expected. In the last few years I rechambered these in 3006, 7x57, 22-250 and 280. I don't remember any shooting larger than 1 1/2 inch at 100 yards and the 22-250 and 7x57 grouped well under an inch.
So after you answer all the customer's questions and he reads some of those articles about Ackley Improved and how much better they shoot, he decides to have you convert his favorite rifle. Now what do you need to know and what do you need to buy? The knowledge part is basic barrel fitting and headspace along with a good understanding about what happens to a cartridge when you pull the trigger. The buy part is easy, A quick call to Brownell's, an Ackley Improved reamer and one headspace gauge. One headspace gauge? Yep, one gauge if you already have a standard set of go and no go gauges in the parent caliber size.
Here is what happens when a high power rifle cartridge fires in a chamber. First the firing pin is released by the sear and travels forward striking the primer. The case is pushed forward as far as the available headspace will permit. The primer crushes and the compound explodes sending flames through the flash hole and igniting the powder. Powder burns and pressure increases inside the case. The increase in pressure speeds up the rate of burn tremendously and the pressure skyrockets. The bullet moves out of the case and goes to the end of the throat. The thin case walls are held against the camber walls by what ever the working pressure of the load generates, maybe 60,000 pounds. When this pressure pushes back it stretches the case back toward the bolt face. With the thin walls held tightly against the chamber wall the thin brass just above the head of the case stretches until the head of the case rest on the bolt face. Hopefully everything is square and true and the bullet enters the rifling straight and exits the crown spinning true and on center.
To make sure this happens with an Ackley Improved we have to know how the parent case in going to headspace in new chamber. If you study the chamber drawings, reamer, parent case and fired case you will see the parent case only contacts the new chamber in a small circle at the base of the neck. With such a small area to headspace on the cartridge has to fit tight. For this reason an Ackley Improved should be set up with headspace less than a standard chamber. When converting to A I the go gauge should be used as a no go and a special gauge ordered or made that is .004" short for a no go gauge. That's why you only needed to order one gauge!
This means rechambering to A I will not be as easy as sticking in the reamer for a couple turns but will require setting the barrel back at least one thread to get headspace too tight before you start to ream. If the barrel is a Remington or Winchester one turn will be .0625 inch so you will have plenty of room to work.
By the time you have removed the barrel, cut back the barrel shoulder the correct amount so the barrel will tighten up with the sights or lettering in the right place, cut back the face of the barrel so it won't be hit by the bolt, recut the cone breech, deepen the extractor cut so it will work and reinstall the barrel in the receiver without leaving any marks on the barrel or receiver you will have earned your pay.
The Ackley conversion is not a small job and should not be attempted till you are competent in all phases of barrel fitting. Just remember "a happy customer will tell a friend but an unhappy customer will tell ten so do good work!