I have long wondered about P.O. Ackley’s claims about bolt thrust. I have started some tests to see if more modern tools can tell us more about bolt thrust than Ackley was able to learn from his test. On page 139 of volume 1 of Ackley’s “Handbook for Shooters and Reloaders” he discusses pressure, and I quote, “The usual method of measuring pressure is by means of crusher cylinders, so as to measure the pressure at right angles to the axis of the bore about midway along the body of the chamber. This method gives a fair idea of the actual chamber pressure exerted upon the walls of the chamber in all directions, but bears little relationship to the amount of this pressure actually transmitted to the bolt or breech block in the form of thrust.”
Ackley believe that minimum body taper reduced bolt thrust. So, I set out to see if it does or not. First I set up a 30 caliber barrel in a test fire jig. I started with a 30-30 WCF chamber, firing factory ammo. Utilizing the Pressure Trace system I installed a transducer on the barrel to collect pressure data.
We also mounted a transducer over the recessed breech in our barrel to see if we might measure longitudinal stretch. This did produce measurement, but only time will tell if they have any value. At this point in the test I suspect that the data we are getting is too flawed to decisively answer any questions.
Its important to lay out some facts at this point.
- SAAMI maximum pressure in the 30-30 Winchester is 42,000 PSI
- Federal Factory ammo was used for the first phase of the test
- Our firing mechanism is adjustable for headspace and the firing pin is adjustable for length so that we can fire with excessive headspace.
We learned something right away; while firing the factory ammo in the 30-30 Winchester chamber and collecting pressure/velocity data, I decided to test various degrees of excessive headspace and see the results. Interestingly, I set the breech with .010″ (10/1000 of an inch) headspace. The fired cases did not move back at all, the primers did back out the exact .010″ distance. Ackley did not test this in his series, so right away we learned that the 30-30, which is considered to be a low pressure cartridge will adhere to a dry chamber under pressure, and the brass can contain the factory level pressure without stretching.
All brass was carefully messured, no head expansion or stretching in length occured. Next we rechambered the barrel to 30-30 Ackely Improved. The same factory ammo was used to fire-form cases. The same test was perform with headspace, even during fire-forming the cases did not move back or stretch in length.
We are now at the stage of the test where we are working up loads to see how much velocity we get at the same pressures as our factory ammo. This is always a hotly debated subject, so it will be interesting to have concrete information to share.
Check back soon to see how things progress with this test. BTW, I am giving you the short version here, the full test and all ballistics will be included in my book on P.O. Ackley when it is published.
Updated March 2017, book has recently been published, “P.O. Ackley, America’s Gunsmith” The completed 30-30 bolt thrust tests are of course included, along with much more.