brayhaven,
No, Ackley did NOT use high-pressure .30-30 handloads in his tests. He used factory loads, as you'll find if you go back and read the test carefully. The rifle was rechambered for the .30-30 Improved during the test, but Ackley continued to use factory ammo. However, he did use heavy handloads in the .250 Savage, which is probably why a lot of people make the mistakes that the .30-30 ammo was handloaded.
But here's an article detailing why Ackley's test, and hence his conclusions, were flawed:
“Bolt thrust is the term used for how much pressure the head of a cartridge case imparts to whatever holds it inside the chamber. This can be a bolt, but it can also be the breech-block of a single-shot, the frame of a revolver, or the face of a break action.
However, the greatest bolt-thrust is created by modern rifle cartridges—along with the greatest misconceptions about bolt thrust. Since the firing of a cartridge shoves the case backwards, many shooters intuitively assume the case head creates the most pressure. They’re wrong, and the common ideas that case shape or chamber lubrication affect bolt thrust are also wrong.
These misconceptions probably arose from a book published in 1962 by famous gunsmith P.O. Ackley, entitled Handbook for Shooters and Reloaders. Ackley was a frequent experimenter, primarily remembered for “improving” factory cartridges with a 40-degree shoulder angle. This technique is so well-known that any improved round with a 40-degree shoulder is known as an Ackley Improved, even of P.O. Ackley never fooled with it.
One example is the .223 Remington Ackley Improved, which didn’t appear in the original Handbook because the .223 didn’t become a factory round until 1964. Fans call it the .223AI, with many claiming muzzle velocities close to the .22-250, especially with 40-grain bullets.
The maximum velocities listed with 40’s for the standard .223 in handloading manuals run around 3800 fps, but .223 AI enthusiasts often report velocities of 4000-4100 fps, an increase of 5-8%. One basic rule of interior ballistics is that potential muzzle velocity in any given caliber is ¼ any increase or decrease in case capacity—if both cartridges are loaded to the same pressure. (I derived this rule empirically from loading data. It’s since been basically confirmed by people who know a lot more about physics.)
According to the 4-to-1 Rule, an increase of 5-8% in muzzle velocity implies an increase of 20% to 32% in case capacity. I just measured the water capacity of a once-fired “standard” .223 Remington case with a Berger 40-grain hollow-point seated to the maximum SAAMI overall length of 2.26 inches. (This is the correct way to measure case capacity, not filling the case to the mouth, since the necks of cases are partially or totally filled with bullets, not powder. A fired case should be used, since new cases hold less water, often much less.) The result was 30.6 grains.
A 20% increase in velocity implies a case holding about 36.7 grains of water, 6.1 more grains than the standard .223 case—a remarkable amount of “improvement,” since the popular.257 Roberts Ackley Improved only gains about five grains over the standard .257 Roberts in a much larger case. A 32% increase in powder capacity would mean a case holding 40.4 grains of water, and the .22-250 only holds around 42-43 grains of water with a 40-grain bullet seated, depending on the brass.
In the real world, of course, the .223 AI doesn’t gain nearly that much powder room. A friend who’s a .223 AI fan was kind enough to give me a formed case, which holds not quite two grains more water with the same Berger bullet seated to the same depth. This is about 6% more than the standard .223 case, and according to the 4-to-1 rule means about a 60 fps increase in potential velocity.
So how does the .223 AI get over 4000 fps with a 40-grain bullet? I discussed this problem with another .223 AI enthusiast, who claimed the reason was the case shape, since less body taper results in less bolt thrust. Upon further questioning, he said P.O. Ackley proved this years ago, in his book.
That evening I dragged out my Handbook, so old and well-read the spine is held together by duct tape, and re-read the chapter on pressure, where Ackley states: “Wildcatters feel that minimum body taper design reduces bolt thrust. This theory tends to be substantiated by results.”
First he describes locking up a Savage 99 in .250-3000 (a very tapered case) by firing a fairly warm powder charge with a 100-grain bullet. When the same rifle was rechambered to an improved version of the .250 with very little case taper, cartridges loaded with the same charge and bullet extracted easily. The improved cases could then be loaded even hotter, but the action wouldn’t lock up.
He next described experiments with an old Model 94 Winchester rechambered to an improved version of the .30-30. Ackley first unscrewed the barrel one thread, whereupon the primers backed out but the case remained in the chamber. He then oiled the case to see what would happen, and upon firing it backed out of the chamber. The same experiment was then repeated with the barrel unscrewed two threads, and the same thing happened. Finally he removed the locking lug and fired the rifle by holding the action closed with finger-pressure, and the case didn’t back out.
Ackley said “the tests described seem to indicate a very small percentage of the CHAMBER pressure was transferred to the breech bolt in the form of thrust,” and claimed they offered proof that case shape has a large effect on bolt thrust. He was wrong.
First let’s look at Ackley’s “experiments” with the .250-3000 in the 99 Savage. I personally did a bunch of experimenting with overloads in the .250 Savage case back when I was younger and dumber, by trying to turn it into a .257 Roberts by adding powder. The rifles were several Savage 99’s, plus a Remington 700, Ruger 77 and Winchester Model 70. The 99s would stick even when the round was relatively lightly over-loaded (if “lightly” can be used in that context), but none of the bolt actions ever required any extra effort in opening the bolt when using loads that would lock up a 99, so something else was causing the problem.
a rear-locking bolt like the one on a 99 compresses significantly when firing an over-pressure round , unlike the front-locking bolt of a 700, 77 or 70. [You can find this information in Otteson's book THE BOLT ACTION.] Any case will back out of the chamber slightly when that happens, and also lengthens slightly due to fire-forming. When the compressed bolt decompresses, the tapered, lengthened .250 case then wedges firmly into the chamber. With the minimally tapered .250 Improved, the case also backs out and lengthens slightly when fired with an overload, but due to the minimal taper and sharper shoulder, doesn’t wedge in the chamber like the standard .250 case. Or at least that seems to be the most reasonable explanation, given the evidence.
As for the .30-30 experiments, Ackley only fired the rifle with factory loads, and the SAAMI maximum pressure for the .30-30 is 42,000 psi. To understand why this experiment doesn’t demonstrate anything about bolt thrust, we need to understand that at a certain pressure, not far above the .30-30's 42,000 PSI, brass cases stretch enough for the case head to press firmly against the bolt face. This is why primers will often be backed out of .30-30 cases slightly after firing, but not .30-06 cases: The .30-06 cases stretch to press firmly against the bolt face.
This indicates that the "yield strength" of cartridge brass is somewhere between 42,000 PSI and the 60,000 PSI of .30-06 factory ammo. Cartridge cases work-harden while being formed in dies during manufacture. The thin neck and shoulder are annealed afterward, so they’ll stretch without cracking, but the case heads are left hardened so they won’t deform or break easily. As a result some primer pockets don’t permanently expand until pressure is over 65,000 psi.
However, even unhardened cartridge brass will withstand 15,000 psi more than the maximum average pressure of a factory .30-30 round. This is exactly why the primers backed out of the factory .30-30 loads in Ackley’s experiment, instead of the case stretching to fit the lengthened chamber. In fact it’s common for primers to back out of cases in lower-pressure cartridges fired in worn lever-actions. I have a Winchester Model 1894 rifle in .25-35 made in 1898, and the primers on factory ammo back out very slightly when fired. In either instance, there’s zero bolt thrust—and the .25-35 cases fired in my rifle aren’t “improved."
The fact that the rifle could be fired safely merely by holding the lever closed doesn’t prove anything about the .30-30 Ackley Improved. The rifle probably would have done the same thing without being rechambered, but Ackley didn’t try it.
The notion that oiling the case results in greater bolt thrust is also faulty, because even the heaviest greases with “extreme pressure” additives don’t retain lubricity above 10,000 psi. Yes, a lubed case with excess headspace will slide back against the bolt face when fired, but once pressures rise above 10,000 psi the case sticks to the chamber.
In the same chapter there’s also a quote from Vernon Speer explaining why the Speer lab used measurement of case-head expansion (CHE) for working up handloads for their manuals. He claimed CHE was more accurate than the copper-crusher pressure guns used in those days, but lab technicians I’ve interviewed say copper-crusher testing can be quite accurate, as long as painstaking measurements are made. The reason it’s not used much anymore is electronic testing doesn’t require nearly as many measurements, so is much faster. Some handloaders still believe in measuring CHE today, when electronic pressure guns and strain-gauges have proven the technique isn’t reliable.
In fact, thanks to technological progress there’s even a way to measure bolt-thrust through highly sensitive films. Charlie Sisk, the Texas gunsmith, has been performing experiments with the Pressure Trace strain-gauge system for a number of years now, and eventually became intrigued with bolt-thrust. He used the Topaq computer services of Sensor Products Inc. to analyze the pressure between the bolt face and case heads in cartridges from the .223 Remington to the .300 Winchester Magnum—including the .22-250, based on the very tapered .250-3000 case.
Sisk applied pressure-sensitive Fujifilm to the heads of the cases, then fired them on his indoor range and sent the cases to Sensor for analysis. The results repeatedly demonstrated that the pressures created by modern bolt-action cartridges (not 1890’s lever-action cartridges), press the heads of cases against the bolt-face with exactly the same pounds-per-square-inch as the rest of the chamber, regardless of case shape.
This isn’t exactly startling news. Ballistic engineers have known it for a long time, but most engineers don’t write popular handloading books. So what’s the source of the magic in the .223 Remington Ackley Improved? I’d long had my own theory, so tested it by handloading some once-fired Winchester-brand .223 cases with deliberate overloads, using Ramshot TAC powder and 40-grain Berger Match Varmint hollow-point bullets.
TAC was used because it’s a fine-grained ball powder, so more will fit in a case than some other top .223 powders such as Hodgdon Benchmark. The Berger bullet was chosen because Ramshot’s on-line data showed a maximum load of 27.3 grains of TAC at 54,170 psi, just below the maximum average SAAMI pressure of 55,000 psi for the .223 Remington.
I started with 29.0 grains of powder, working up in half-grain increments to 32.0 grains, stopping there only because 32.0 grains was all the TAC I could get in the cases, even when tapping the funnel while very slowly dripping the powder into the necks. Then I fired the loads on a 70-degree day in a Thompson/Center Icon with a 22-inch barrel, chronographing the velocity with an Oehler 35P:
29.0 grains—3658 fps
29.5 grains—3759 fps
30.0 grains—3849 fps
30.5 grains—3946 fps
31.0 grains—3997 fps
31.5 grains—4014 fps
32.0 grains—4025 fps
No traditional sign of high pressure showed up until the 31.0 grain load, when bolt-lift became a little stiff and two case-heads showed a slight ejector-hole mark . At 32.0 grains all the cases had definite ejector-hole marks, and lifting the bolt required considerable effort. In fact on two cases the bolt handle had to be tapped open with a wooden hammer-handle. Also, note that velocity increases were also much smaller in the loads over 30.5 grains.
I then tried the 30.5 grain load in a Remington 700 with a 26-inch barrel. The muzzle velocity was 4046 fps, and there were zero signs of high pressure. Adding the 60 fps from the 4-to-1 Rule would result in the muzzle velocities many .223 AI fans report.
Obviously, the 30.5 grain handload produces far more pressure than the 27.3 grain maximum in Ramshot’s data. I didn’t have the load pressure-tested by either Western Powders or Charlie Sisk, but according to one of Homer Powley’s formulas the pressure would be at least 65,000 psi, 10,000 psi over the SAAMI limit for the .223 Remington.
Are such loads safe? Obviously, SAAMI’s maximum .223 pressure of 55,000 psi is lower than the yield strength of even unhardened cartridge brass. It’s also obvious that some handloaders have been firing .223 AI handloads at much higher pressures for a long time. A smaller case-head produces less bolt-thrust, due to less area pushing against the bolt’s face, so maybe they’re safe, but the velocities they’re getting aren’t due to the magic of an “improved” case. Instead they come from the tradition of working up wildcat handloads until the rifle and brass show signs of distress, then backing off slightly. The result is extra pressure and extra velocity. It’s that simple."
After this article was published, I was contacted by two different engineers who objected to the proof of the Fujifilm pressure tape, mostly because they had swallowed Ackley's "evidence" years ago. One insisted the results with the film were flawed because the headspace in the test rifles was too minimal, compressing the tape. The other engineer insisted the headspace was obviously too much. Which just proves the old definition of two engineers in a room is "an argument." Neither one could provide any evidence to contradict the results of the tests.
Also post-publication, I discovered why the .223 was once reported as producing velocities almost as high as the .22-250 with far less case capacity. It turned out a major bullet company came to that conclusion after strain gauge tests of the .223 AI, where velocity were very high but pressures in the 60,000 PSI range.
But apparently, the person who ran the tests didn't understand that strain-gauge pressures run somewhat lower than piezo-electronic pressures. When the same .223 ammo was retested in a piezo lab later, the magic velocities of the .223 AI disappeated--because they were due to much higher pressure than 60,000 PSI.
