Ackley Improved Cartridge Headspace

Reader Patrick Ryan pointed out a probable error in my June 2019 column on the .257 Roberts Ackley Improved wildcat that I’d like to address here. I had stated that when Ackley developed the improved version, he “maintained the shoulder headspace datum of the parent factory round.” I recognized Patrick’s name as someone who had worked for Redding, and I’d previously talked with him several times. He stated that the Ackley Improved cartridge “…shared the same dimension (as the parent round) from the boltface to the junction of the shoulder and neck. This is what allows the parent cartridge to be fired in the improved chamber.”

I had his email address, so we messaged back and forth about this and other shooting topics, and I could only conclude that my understanding of AI/parent cartridge headspace was flawed and that I had misspoken.

The editor was concerned because another “The Reloader” column on the .30-06 Ackley Improved was about to be published, and it contained a comment similar to the one described above. This time I’d stated: “One key feature of the AI chamber is that it maintains the same headspace as the parent cartridge. That means factory ammo can be safely and reliably fired in a rifle chambered for the AI cartridge.”

I promptly pulled out various reference materials, including cartridge illustrations from Wolfe Publishing (1993) and RCBS (circa 1986) and Ackley’s two-volume handloading manual (1962) to investigate the situation. Comparing illustrations of the .30-06 standard dimensions to the AI version seemed to refute Ryan’s observation.

Now What Could I Do?

I decided to call upon the knowledge of two longtime reloading mentors (ST columnist Allan Jones and former ST Rifles/Reloading Editor Rick Jamison) and the Nosler R&D Manager Mike Lake (Nosler sells factory-loaded .280 AI ammunition and rifles chambered for .280 AI). Allan promptly pulled up SAAMI drawings of the .280 Remington and .280 Ackley Improved. The latter is the only Ackley wildcat that has “official” status. He compared the shoulder-neck dimensions and promptly remarked, “They don’t match either.” Rather than help my situation, that puzzled me all the more.

I pulled up the .280/.280 AI SAAMI specs and noted they have the same Basic (i.e., theoretical) headspace datum (the place where a 0.375-inch-diameter ring gauge should touch the shoulder of either cartridge). On the .280 Rem. (with a 17.5-degree shoulder angle) that is 2.1042 inches from the cartridge base. The corresponding .280 AI dimension (with a 40-degree shoulder angle) is 2.1437 inches. That’s nearly forty thousandths of an inch longer. And the neck-shoulder dimension of the .280 Rem. is 2.1993 inches compared to the .280 AI’s at 2.1795 inches. That means the standard round coordinate can be nearly forty thousandths of an inch longer than the AI cartridge’s.

When I posed the same inquiry to Rick, his response was, “A standard round headspaces at the base of the neck in an Ackley chamber, at least enough to fireform the case. Just fire standard rounds in the Ackley chamber. If you have an Ackley chamber, all you need are loading dies and you’re in business. The setup is simple and virtually trouble-free. Ackley sold both chambering jobs and dies. As a customer, it’s an inexpensive way to get higher performance. You don’t need a new barrel, and rounds will feed and fire without further action adjustments or other gunsmithing required.”

I pointed out that the Wolfe Publishing drawings indicated the neck length of the .30-06 case was 0.388 inch and the .30-06 AI case neck length was 0.419 inch, meaning the parent round’s neck was thirty-one thousandths shorter than the AI version. We concluded that sounded reasonable. The standard round’s sloping shoulder could never touch the AI chamber shoulder before firing. So the neck-shoulder junction of the parent cartridge must reach the AI chamber neck-shoulder with at least a slight crush so it would reliably fire.

While discussing the matter with Mike, he reminded me there were many versions of the wildcat “improved” .280 Remington, including several labeled “Ackley Improved.” To add to the confused situation, even the tooling makers’ drawings differed! Nosler decided to follow Mr. Ackley’s intent as closely as possible—to design the chamber so a full-pressure .280 Rem. factory round could be safely fired in SAAMI-compliant .280 AI rifles.

---

Nosler compared the maximum and least material conditions, i.e., assessed the effect of the required tolerances that must be allowed for both the cartridge and chamber configurations. Accordingly, the company determined the amount of crush versus clearance at the neck-shoulder junction could range from 0.009 to 0.016 inch respectively. Statistically speaking, rarely does either extreme occur because the multiple manufacturing tolerances will average out. So that’s why a .280 Rem. factory round typically fits into the SAAMI-spec .280 AI chamber with just a slight amount of crush.

I recalled that when my gunsmith, John Gallagher, rechambered my Savage .30-06 rifle he told me he used a .30-06 go gauge as the Ackley chamber’s no-go gauge. He had learned that trick from Dave Manson, maker of chamber reamers and other precision gunsmithing tools. The .30-06 AI go gauge is precisely machined to measure 0.004 to 0.006 inch shorter than the parent case go gauge. He recommends setting the barrel back one turn when rechambering to ensure proper headspace. John did so, and my rifle reliably feeds and fires standard .30-06 and .30-06 AI rounds interchangeably.

In either case, as Patrick pointed out, my first statement was wrong. The headspace datum location on the case body of a standard round compared to an AI version can’t be the same because the shoulder angles are so different. However, my similar statement in the subsequent .30-06 AI column is less specific, so it more correctly describes the relative headspace between the parent and the AI versions.

I don’t know if I’ve completely cleared up the misunderstanding, but I have a better handle on the topic. I guess you’re never too old to learn something new.