Experimenting with Bolt Thrust

Experimenting with Bolt Thrust
Bolt Thrust - pressure and velocity data
Collecting pressure and velocity data at the range.

Since the post-WWII years, if not before, there has been an ongoing argument concerning whether breech thrust (bolt thrust) is reduced by the improved case design. P.O. Ackley has certainly influenced the argument. The definition of an improved case is pretty simple. The case body is blown out to minimum body taper, which is described by Ackley as 0.0075 per inch taper. Shoulder angles between 28 and 45 degrees are normally considered to be improved, although it could be argued that any shoulder sharper than the original parent case is improved. Finally, an improved design allows the firing of a factory cartridge in order to fireform the brass for the new design.

Shoulder angles between 35 and 40 degrees seem to provide the advantage of minimizing brass flow without negative effect. When the shoulder angle is greater than 40 degrees, brass is unnecessarily hard to form and chamber reamers do not last as long. Headspacing becomes much more critical with a sharper shoulder because there is less taper, making it harder to hit the correct measurement. Also, sharp shoulder angles do not feed as smoothly as more tapered ones. When the shoulder angle is less than 35 degrees, brass flow becomes more of an issue. There are some cartridges, like the .220 Swift Improved, which do not receive any real improvement in velocity, but are popular because they improve brass life by arresting stretch, thereby increasing brass life.

It is not unheard of to measure breech thrust, however the cost of tooling for such testing made it impossible for the purpose of writing this book. However, a method of recording breech trust was necessary in order to go beyond the somewhat subjective experiments that P.O. Ackley wrote about in Handbook for Shooters and Reloaders Vol. I. There Ackley used a Model 94 Winchester because, as he stated, “We often hear that the Winchester Model 1894 action was designed for low pressures and is an action which could be described as ‘weak.’” The purpose of his experiment with the ‘94 was to prove that the improved case design minimized bolt thrust; that the brass will support and contain some pressure; that oily chambers increase bolt thrust; and finally, the notion that actions are designed for specific pressure ranges is a fallacy.

Bolt Thrust - Firing mechanism
Firing mechanism, adjustable for headspace and firing pin length.

Our Test
The Pressure Trace (a product of Recreational Software, Inc.), was used to measure the chamber pressure for all ammo tested in this chapter. The goal of this test was to repeat Ackley’s experiments with the .30-30 Ackley Improved, but to increase the value of the data collected by taking measurements of the thrust against the bolt face. This experiment is repeatable. The author designed and built a custom jig to hold a .30 caliber barrel with a universal breech plug to allow for adjustable headspace, and to accommodate the strain gauge utilized by the Pressure Trace. The firing pin had to be designed to allow for headspace adjustment, too.

The initial tests were done with factory loaded ammunition in .30-30 Winchester to provide a baseline comparison. The second wave of tests was performed after the barrel was rechambered to .30-30 Ackley Improved. The breech of the barrel was turned to 1.050 inches so that wall thickness would be thin enough to provide good data with the relatively low-pressure factory .30-30 Winchester loads. We also left the wall thickness in the area of threads as large as possible so that it would be less likely that the breech would expand, allowing the breech mechanism to move rearward and partially nullifying our test results.

The bolt thrust test had the following goals:

  • Determine empirically if the cartridge case does indeed contain some pressure at factory levels.
  • Determine difference in bolt thrust between factory and improved designs, if any.
  • Determine if chamber pressure can be increased over factory with the same bolt thrust regardless of pressure.

For the .30-30 AI, run pressure up to the point where the brass separates, and compare results to mathematical predictions.

Oil cases to see if bolt thrust is increased with the same load as Ackley stated.

Extend the experiment to other cartridges.

Bolt thrust - prescale film
Prescale film in place, note the hole in the film for the firing pin.

.30-30 Test
In this first stage of testing with factory ammunition, we fired 60 rounds to work out technical issues, and insure that the test rig would work for the designed purpose. While firing these early test rounds we also checked to see if we could fire rounds with excessive headspace as this was part of Ackley’s earlier tests. We first fired a batch with .010-inch headspace. The cartridges were pushed forward so that the rim was in contact with the breech of the barrel. When fired, the primer backed out .010-inch to take up the headspace, the case stayed fully forward and did not measurably stretch.

We then experimented and found that the maximum amount of excessive headspace we could generate without the primer failing was .046 inches. When we exceeded this amount the primer ruptured and left us with lots of pieces and carbon in the breech gap.

Thus, before we even started the test in earnest, we had proven that the cartridge case of the .30-30 Winchester can contain all of the pressure of a standard factory load without stretching and that the primer is actually the weakest part of the cartridge. That bit about the primer is no real surprise to you reloaders.

Bolt thrust - 30-30-Win
Ammunition used to provide this reference sample, Federal Power-Shock.
bolt thrust - 30-30 Ack- rechambered
Ammunition used to provide this reference sample, Federal Power-Shock.

bolt thrust - 3030-Ack-ImpThe reason the neck and shoulder are dimpled in the photo this chapter is simple. Gasses slipped back around the neck. It is likely that the pressures were higher in the barrel than in the case once the primer failed — venting gases into the breech gap. So the higher pressure gas in the barrel was seeking an outlet around the case neck.

Once we were confident in our results, we then recorded the data listed in the table below by firing 10 rounds of each load. Average readings are used to help keep all comparisons apples to apples. The factory ammo did produce some pressures over 40,000 psi, which means it is loaded close to the safe limit set by SAAMI.

A product called Fujifilm Prescale offered by Sensor Products, Inc., of Madison, New Jersey, was used to record bolt thrust. Prescale is a Mylar-based film that contains a layer of tiny microcapsules. When pressure is applied to the film the microcapsules are ruptured, producing an instant and permanent high resolution image of the pressure variations across the contact area. The film we used was .004-inch thick and comes in varying pressure sensitivity. By placing the film between the case head and the bolt face we are able to take a reading of the exact and true bolt thrust in real time. The film can also be sent to the company for computer analysis, which will reveal the exact pressure exerted, including detail of where the pressure was applied and where it was less intense.

Bolt thrust - seating depth
Note how easy it is to spot headspace from the primer protrusion on a .30-30. Case on the left has zero headspace.

Seating depth in all the loads listed for the .30-30 Ackley Improved here was 2.535 inches, seated to the canelure of the bullet. Note that the last load in the table above, 35.5 grains of IMR 3031, is a compressed load, and there is no room for any more powder. This is approximately 140 fps faster than published data for this powder and bullet weight in the standard .30-30 WCF, and there is clearly not enough room with this powder to get into pressure problems.

The 35.5 grains of H322 was used as it produced factory level pressures in the .30-30 AI chamber. Excessive headspace of .010-inch was set. When fired, the case did not move back, the primer backed out and marked the Prescale film. Edges of the film show color only because the film was cut to fit the case head. The color or light readings around the primer pocket are “noise” from the vibration of the bolt during the firing of the mechanism. This was proven by dry-firing the mechanism, during which similar marks appeared in the film.

bolt thrust - 30-30-ackley-over pressureWhen you study the table following here, it will be obvious that we have exceeded the SAAMI pressure limit of 42,000 psi with H322. This powder allowed us to get more powder in the case because of its smaller granules. 35 grains would be the safe maximum in our test barrel if you were staying with the SAAMI pressure limit, and at 2,600 fps we are nearly 400 fps past the published data for the same powder and bullet weight in the .30-30 WCF. Of course, this is only true in our test barrel, it would be necessary to use normal load development for any individual firearm, as we were able to generate far more pressure than is advisable in a .30-30 AI under normal conditions.

Those loads that exceeded the 42,000 psi limit were tested for two purposes: To see how much pressure could be generated in the .30-30 AI case, using powders that are appropriate in burning rate for said case. And to determine at what point the brass will yield and stretch with excessive headspace.

bolt thrust - 30-30-ACK-4895

bolt thrust - 30-30 Ackley improved
The above two tables show that the .30-30 Ackley Improved delivered an additional 180 to 200 fps with these powders in our test barrel. The pressure curve on the H4895 loads had an undesirable form in the .30-30 AI: instead of the pressure curve rising early and tapering off, we found that it had a second pressure spike much higher than the first just before the bullet exited the barrel. For this reason alone I would avoid this powder in a .30-30 class cartridge. *Exceeds SAAMI Maximum.

We started with 0.000-inch headspace as before. The oil on the cases did not allow them to adhere properly to the chamber walls under pressure. Consequently they moved to the rear and applied full pressure or bolt thrust to the bolt face.

This is a stark comparison to the earlier test where the dry case was able to adhere to the camber wall and only the primer backed out against the bolt face.

Bolt thrust - cases
The brass from our hottest loads shown here, were intentionally shot with excessive headspace. Expansion of the brass to full chamber diameter is normal and can be seen on the fired case. The expansion line on the bottom case is where you would expect to see case stretch if it were present. Looking across at the sectioned case, there is no sign of stretching of the case wall, i.e. no thinning of the case wall.

So, was Ackley right about his findings?
Yes, but he may have missed a point or two.

Since .30-30 brass is thick and pressures are low relative to brass strength and case capacity, with most appropriate powders pressure is not a big problem. To be fair, we did find some powders that will develop pressure far beyond SAAMI levels for the .30-30 AI case. Because the brass is so thick, it actually cannot stretch and cause head separations due to excess headspace. In that respect the .30-30 is not a good choice for Ackley to prove that improved designs handle pressure better.

However, Ackley used the .30-30 because the ‘94 Winchester action had been labeled weak. In this respect, Ackley did prove that the ‘94 can handle anything the .30-30 or .30-30 AI can dish out, without any question.

Editor's Note: This article is an excerpt from P.O. Ackley: America’s Gunsmith.


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