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?

Bobby wrote, "Breech thrust is NEVER increased or decreased in relation to the pressure generated by the cartridge."



This seems completely counterintuitive. If pressure is exerted in all directions at once, and 'all directions' includes rearward, then breech/bolt thrust should be directly proportionate to pressure, all things being equal. Go to the two extremes--one load producing enormous pressure and one load barely a squib. How could they possibly produce equivalent breech thrust. That's like saying that they would produce equivalent velocities. I am a newbie, so maybe I'm completely wrong. Please explain.

Cris F; Just read Ackley again. To begin with since the forward part of the case stays forward, improved or not, it can't effect the base thrust. I don't doubt Ackley for a moment, but I question his analysis of the information.
The two chambers probably had different headspaces. It was Ackleys approach to use minimum headspace with improved chambers. The free motion in the standard chamber could have contributed to "lock up". This is inforced by Ackleys
statement that lockup "is more pronounced in loose actions".
Isn't it wonderful that the more you learn the less you know?.
Take Care!

Breech thrust is NEVER increased or decreased in relation to the pressure generated by the cartridge. In other words, any given pressure is exerted in all directions equally. When a cartridge is fired, the pressure generated by the burning powder exerts pressure equally in all directions. Even if the case did somehow "grip" the chamber walls, the brass is not strong enough to not obtruate and extend to fill the chamber. The chamber contains the pressure, and the brass is only the gas seal, without the chamber, the brass would fail for lack of support. So no matter what condition the chamber is in, oiled, rough, or smooth and dry the bolt face would still see the exact same amount of pressure.

irv and Clark,
Thank you for indulging my curiousity in this whole thing. I started out with the "pat" Ackley reference...and now find myself rethinking everything.

I just ran across this in my "reading room" yesterday and thought it might have bearing on the discussion;
Comments?
BTW, I'll be leaving on a trip Monday nite and may or maynot have access to the web...in case I don't respond.