Has anyone studied this? I'm wondering if breech thrust would increase or decrease when fireforming in an ackley improved chamber. The extra space in the chamber seems as though it would lead to diminished pressure, however, if the case walls don't immediately grip the chamber walls, breech thrust would be increased. Can anyone offer some information about this?

I have fire formed 100 cases for my .257 Roberts AI and have had no indication of increased bolt thrust. I'm not sure how to measure it, but the bolt opens easily. The pressure generated by the fire forming load I use can't be too much as it is only 11.0 grains of Unique and enough Cream of Wheat to fill the case to the neck.

..again not exactly what you're asking, but I thought it was pretty interesting nonetheless.
From VarmintAl's.

PO Ackley wrote about this. From "Handbook for Shooters and Reloaders" volume 1.



"Wildcatters feel that the minimum body taper design reduces bolt thrust. This theory tends to be substantiated by results..."



One experiment and and written about by Ackley was to run up pressures in a Savage 99 in 250/3000 with 36gr of Hi-Vel#2 and a 100gr bullet which froze the action. He had previously established a max load of 34.5gr for 2900fps at 52,000psi.



He then reamed out the chamber for a reduction in body taper (ala Ackley Improved). He then again ran up loads starting with the 36gr/100gr load that had previously froze the action. Extraction had become extrememly easy.

"Since case capacity has been increased, the velocity with show about a 2% reduction. To continue the test, the powder charge can be increased a grain at a time until trouble develops....This time extraction trouble seldom shows up before something else gives, which is usually a swollen chamber indicating that the bolt thrust which caused extraction troubles with the original chamber has virtually disappeared while the chamber pressures has been increasing.... the loads can be once again be increased to a point of loose primers before any extraction trouble shows up." (in this test he got to 42gr of HiVel#2 and the 100gr bullet for a velocity of approximately 3400fps.)



If we accept difficult extraction as a sign of increased bolt thrust, and loosening primers as a sign of overpressure, then Ackley reached higher pressures with his "Improved chamber" before he reached similar backthrust that bound his action when it previously had a standard chamber. Sounds like the "improved" chamber reduced backthrust.



"By studying these pictures it can be easily seen that the brass cartridge case WILL withstand at least SOME pressure, that the minimum body taper takes the "load" off the bolt or locking system, that oily chambers increase the bolt thrust..."



"Using pressure figures are misleading. They do not take into consideration the design of the cartridge nor the method by which the pressure figures are obtained...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. The English method actually measures actual thrust against the bolt by inserting a crusher disc into a specially designed bolt in such a manner that the disc receives and measures the actual thrust which in turn is converted to breech pressure."



Dan Lilja wrote an article a few years back on calculating bolt thrust. I'll see if I can dig that up, as well as Varmint Al's illustration of pressure distribution and what happens to a case upon firing.

Thanks Chris F. I appreciate how much time you took to answer this.

What I'm doing is rechambering a Model 336 30-30 to .307 Win. I figured that if I had the chamber improved to .307 AI, then I should be able to reduce pressures and breech thrust by using normal .307 win loads. Is this correct thinking?

I'm also wondering, can I get away with fireforming using factory ammo or would the cartridge base slam into the bolt face before the case could expand completely? Am I completely wrong?