Home Forums Hunting & Shooting Gunsmithing Neck/shoulder junction on Ackley Improved cases

Would someone know how far below the standard cartridges' neck/shoulder junction Ackley would have his neck/shoulder junction on his improved cases so as to get a crush fit to fireform the standard cartridge into the Ackley Improved chamber? I'm just working out the specs for a reamer on a wildcat with a 30 degree shoulder and want to be able to headspace a standard cartridge with a crush fit into the improved chamber so as to fireform it, but minimize the amount of thicker brass from the shoulder forming part of the neck on the improved case. Thank you.

.004 is the usual. -Al

Thanks Al.

Not sure you and Al are on the same page. You seem to be asking where is the neck/shoulder junction on N AI case versus a standard case. Al gave you the crush on a standard case when chambered in an AI chamber. Those two are not the same thing. Then again, I might be missing something myself.

If the neck/shoulder junction on the improved case reamer print is .004" closer to the bolt-face than on the standard cartridge reamer print, then a crush fit of the standard case into the improved chamber for fireforming should work. I wasn't sure whether Ackley went .010" or .005" etc. Not sure if this clarifies my question or not.

The thing is, the neck/shoulder juncture varies with neck diameter, shoulder angle, and the radius at the transition. This means that -.004" on the gauge may or may not produce any crush on the standard factory case. I usually chambered AI's at -.006" and even that did not always produce crush on new brass. GD

The thing is, the neck/shoulder juncture varies with neck diameter, shoulder angle, and the radius at the transition. This means that -.004" on the gauge may or may not produce any crush on the standard factory case. I usually chambered AI's at -.006" and even that did not always produce crush on new brass. GD

Al and greydog are right. The .004" is an estimate, but in reality it all depends on the brass, and even that varies. I once chambered an old barrel for fireforming, used the .004", and had 100 new cases to fireform prior to a prairie dog hunt. Most closed with feel, so one would expect. Some closed without a feel and some I had to push hard to close the bolt. Same lot of brass. Next time I will go .006" and seat a bullet long as insurance.

BTW, a 6PPC does the same thing. A chamber at zero headspace won't quite close on a 220 Russian case, unless you run it in a sizing die to slightly push the shoulder. If you chamber .002" deep, they close fine.

There is also the chamber headspace tolerance to consider. The basic crush should be determined between your chamber and your brass. In other words, fit the cartridge crush to your components.

Yes....in the end, the brass becomes the biggest variable. Fortunately, either a bit 'short' or a bit 'long' are easily dealt with. A firm jam into the rifling and a lightly lubed case on the first firing helps, too. -Al

I've only headspaced an AI cartridge once, using a go gauge for the parent cartridge as a no-go gauge, installing a Savage prefit. I first set the barrel to minimum headspace, then turned it in another appropriate .004-.005", then tried the go gauge, which wouldn't allow the action to close. Then I tested a few new cases from three brass makes, Winchester, Hornady, and Sig. There was a slight crush fit from two makes, and a bit more resistance with the other, the Winchester. I called that good, and used the COW method to form the cases. The results have been everything I had hoped for.

I guess I’m not grasping the OP’s question. He seems to be asking the position of the neck/shoulder junction of a std cartridge versus an AI cartridge and relating that to crush fit. I’m thinking headspace, and therefore amount of crush on a standard case, is properly set by moving the datum .004 to .006 rearward and case/neck junction isn’t the determining factor?

Correct. The neck/shoulder junction is not the datum line used for headspace on a SAMMI chamber. -Al

I just checked a 220 Swift print and also a 220 Swift AI drawing. The standard case was 1.9057" and the AI 1.9004", a difference of .0053". So .004" to .006" should work fine. Thanks everyone.

When you run those same numbers for a .223 Remington and .223 Remington AI what do you get? How about .22-250 and .22-250 AI? Just trying to understand your calculations a little better.

The bolt-face to the neck shoulder junction. I don't have access to the prints you mentioned.

You can look up reamer dimensions here:

https://4drentals.com/products/reamer-rentals/rifle/

I think you’ll find it interesting to compare the neck/shoulder junction for the cartridges I mentioned.

The shoulder juncture distance will vary according to neck diameter. This means it will vary between a fired case and a sized case. In some of the chamber drawings for AI cartridges, the dimensions are those generated by CAD programs while others represent measurements taken from cartridges. Some drawing are from various reamer makers while others are from Ackley himself. Some of Ackley's drawings have errors, and, in one case, the error is glaring. GD

I believe the dimensions on the 4d reamer rental site are from reamer blueprints. One can easily compare, given correctly cut chambers, the difference that will result in a std caliber versus an AI chamber when cit with the reamers supplied by 4d. And one can easily see there is no uniform neck/shoulder junction difference between a standard caliber and an AI. At least in the case of the reamers 4d rents out.