Barsness's article on the Improved cartridges in the October 2013 issue of Handloader Magazine was quite well done and quite informative.
I have to hand it to Barsness, and Sisk especially for doing what I have always wanted to do..... actually measure the force applied to the frame.
Overall and for general purposes today, they are right as best my experience tells me, but I don't think they have dug quite deeply enough into it.
He developed his theses well enough in regard to case shape, extraction, and velocities, but there are a few other areas that could be developed perhaps in a follow up article.
Using the .223 Rem. v. .223 Rem. Improved to prove a point about case shape lacks a bit of proving or disproving the straight wall case concept's validity, as commonly held, conclusively.
In this instance there is only about .010" difference in shoulder diameter between the standard .223 and the Improved. That degree of straightening the chamber wall has minimal effect.
The difference is next to nothing compared to the more extreme differences Ackley was dealing with back in the 1940s when highly tapered cases were most common. More in this regard later in this piece.
There is little or no doubt in my mind that the extreme velocity gains reported with Improved rounds is in fact the result of running pressures much higher than standard. The long standing debate among those shooting the .30/06 Improved is a good example. The ballistics labs don't show much gain potential with the Improved version, while shooters with little knowledge of what the pressures being generated are put the .30/06 Improved almost on par with .300 H&H Magnum. I believe the ballistics labs, but that does not mean I personally do not or would not push the limits myself based on primer pocket/case head life or case head expansion measurements when working with a modern bolt action or Ruger No. 1 type fixed barrel gun.
In strong fixed barrel guns like most modern bolt actions, using primer pocket life as the yardstick for measuring pressure is still valid in my opinion, even though pressures are likely well above SAAMI standards. The cartridge case is like a fuse or circuit breaker. It is and in some regards is intended to be the "fuse", the weak link that sets the upper limits of pressure.
A strong modern action will generally handle whatever the brass case head will handle. The case head fails before the action fails and fails with more catastrophic results compared to simply blowing a primer pocket.
Barsness notes that Speer used case head expansion as the criteria for maximum pressures in the early days. All you have to do is compare reported velocities in the older Speer manuals with everyone else's velocities published today to see Speer's velocities pushed the upper limits harder than most of us want to go to. Using case head expansion to determine maximum pressure limits in strong actions is valid, though not necessarily the most prudent, not for everyone, and not for the break open guns. The industry's more technically refined modern methods of measuring pressure are:
1) the safest, and
2) keep shooters where they should be with the stretchy break open guns.
In other words, with anything larger in diameter than the .223 Rem. case head fired in the break open guns, stick with the books and DON'T use case head expansion to determine maximum pressures especially if you are prone to pushing the limits.
Overall, and in spite of my own personal association with P.O. Ackley for 10 years, I have to accept what Barsness demonstrated in his article, but on the other hand would like to see him dig just a tad deeper. Some of my own experiences chambering and test firing barrels for the break open guns suggest the highly tapered cases do set back against the breech somewhat differently from the straighter walled cases.
One example that sticks in my mind from back in the early 90s working up max loads in a Contender barrel I chambered for .219 Zipper, an antiquated, highly tapered case. In contrast to the Zipper, the straighter walled chambers for the Contender based on .444 Marlin brass for example will let you continue to increase pressures past where you should be based on my "fly by the seat of my pants" pragmatic methods also validated by the extensive work Don Shearer did in the late 90s.
With the Zipper, powder charge increases went "normal" with the Contender frame unlocking normally, then WHAM, just another grain or so produced hard unlocking. Whether it was due to the inherent nature of the thinner brass or the shape of the chamber, I do not know. But the sudden occurrence of hard unlocking warrants further examination. There does appear to be a difference in how the tapered case sets back, or more correctly maybe at what point in the pressure curve the case sets back compared to a straight walled case with the same thickness. That should prove to be a worthy thesis to follow through with a valid testing procedure.
What Barsness did not mention, and which IS an important factor, is the surface finish in the chamber in regard to the case expanding in the chamber and clinging to the chamber wall, thus making the case contain all of the pressure load like a cork in a bottle.
Eons ago, I did some experimenting with surface finishes in a .35 Rem. Contender barrel fired with factory ammo and with NO extractor in the barrel. With no extractor in the barrel, I could of course readily measure where the case head was before and after firing. Polish the chamber with a more coarse finish, and the case stayed down below the end of the barrel where the firing pin had driven it. Polish the chamber to a smoother finish, and the case would set back against the breech every time. Roughen the finish, and it stayed below. Polish it smoother again, and it set back every time like before. Ie, it was highly repeatable. For the case to stay forward in the chamber, contain whatever pressure the brass wall will stand, and not slide back against the breech, there has to be enough roughness in the chamber for the case to adhere itself to.
What you can "take home" from this and what Barsness depicted well with the examples he gave in this regard, is that up to a certain pressure level, the case does in fact take all of the pressure load, and the action (or frame) does nothing but hold the barrel and fire the primer. No significant force is applied to the breech face other than the force from the protruding primer.
The case moving back or stretching back occurs somewhere around a guesstimated 40,000 psi, at least factory ammo with thinner brass from earlier vintages of cartridge development. On the other hand, my own experiments with the more modern and thicker .308 Win. amazed me when repeatedly the case did not move back to even TOUCH the breech face until powder charges were worked up to within a grain or so of published book maximum. .308 Win. is every bit as straight walled as the Ackley Improved configuration. Was the case holding all the pressure up to nearly book maximum due to the straight wall or the thickness/strength of the brass I cannot say for a certainty, but most likely it is the thickness of the brass having the most effect.
Likewise, at one of the last long range handgun shooting clinics that Don Bower put on, I witnessed quite a number of extremely hot 7mm Super Bower loads WAY too hot for even a bolt action fired repeatedly from a stainless Contender frame with NO indication of loosening the barrel lockup or any other sign of stretching the frame. 7mm Super Bower is simply a 7mm-08 Improved made from necking down and fireforming the extremely thick .307 Win. brass. Section a .308 Win. case and a .307 Win case. Back toward the head you will find the .307 Win. case is about 1/2 again thicker than the .308 Win. case. It appears the only reason the frame was not destroyed can be attributed to the extremely thick and tough .307 Win. case. Examples such as this are one reason why I strongly recommend the .307 Win. case in the break open guns, particularly the Contender (but only with the shoulder blown out to 40 degree and the chamber deeper such that it both decreases chamber pressure and prevents a rimless .308 Win. round from firing in it).
My take on the Ackley Improved rounds is this, without contradiction of Barsness's article but rather adding to it:
1) Their real purpose for existence is the increase in case volume that lets you have enough room for a notch or two slower powder burning rate to affect an increase in velocity, which by the way normally means you need a couple more inches of barrel length to realize the gain from the slower powder. Said another way, where medium burning rate powders deliver their payload well in a 22" barrel, the slower powders need a couple more inches to milk the velocity potentials from them. In regard to volume, in plainer English, if you can't cram enough slow powder in a standard case to realize maximum potential velocity from a standard round, improving it lets you do so.
2) Especially in the springy break open guns where headspace and consistent primer ignition are more critical, the sharper 40 degree shoulder is much less susceptible to collapse of the shoulder when the firing pin drives it forward in the chamber. Experiments I have done and you can very readily do yourself show that the .308 Win. case shoulder for example is crushed back about .003" by the impact of the firing pin. In contrast, the cases with the 40 degree shoulder hardly budge .001". Thus I favor the 40 degree shoulder for its stronger structure. Likewise, I prefer something with a rim, which does not "give" when the firing pin whacks it.
3) The 40 degree shoulder does have an effect on how powder is burned and the pressure curves are generated. The effect is most prominent with larger volumes of powder combined with small bore sizes. The effect decreases with increases of bore diameter and/or the volume of powder is decreased. To better understand the contrast, at one extreme the .22 Cheetah (aka .22x243 Win Improved) benefits more from the 40 degree shoulder in terms of how the powder is burned; whereas, the .35 Whelen gains more in terms of consistent primer ignition and reduced case stretching from the stronger structure of the 40 degree shoulder, but less from how the powder is burned.