what are the principles behind an ackley chamber?


August 6, 2010, 12:13 AM
i've never messed w/ any ack imp cartridges. never had a need or desire to, but i'm curious...

is an ack case just a regular case w/ the taper blown out, the neck moved forward some, and sharpened some? how come ack cases don't have headspace issues?

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August 6, 2010, 12:20 AM
Because on an ackley cartridge the shoulder is pushed out but most importantly the shoulder terminus is set back slightly to provide a slight crush fit and thus headspace controll with factory loads.

To do an ackley from an existing chamber the bbl MUST be set back slightly

August 6, 2010, 07:29 AM
so can a similar effect be obtained another way? i.e., what if you had a rifle in a standard chambering that has a little too much headspace? can you take the approach of creating/forming a second shoulder, crush fit that, and then fireform the cartridge?

August 6, 2010, 08:51 AM
Ackley's basic goal was to obtain as much powder space as possible while keeping the costs down. Doing that required a minimum body taper and a sharper shoulder that still started at the original neck junction, only the rearward part of the shoulder is moved forward by the angle change. This was done so a user could simply chamber a factory round and use it if he needed to do so, the cartridge would be held in place by the neck-shoulder fit before firing.

It might help to set the barrel back a thread before chambering but it certainly wasn't necessary, by Ackley's specific design goals.

Ackley chambers have the same headspacing qualities of any other cartridge. It's certainly possible to sufficently control cartridge length in longish chambers by such handloading methods as you suggest.

August 6, 2010, 09:39 AM
I hear the cases don't stretch as much due to shoulder angle.
I ran into 1 or 2 guys locally that have AI chambers.

August 6, 2010, 10:55 AM
In theory both shoulder junctures move, the shoulder/neck juncture moves back, as a results the neck on the Ackley version is longer than the parent neck. then the shoulder juncture at the end of the case body and beginning of the shoulder is moved forward, as a results the body of the case on the Ackley is slightly longer, this causes a problem for those that make this stuff, there is a point on the shoulder that does not move meaning the DATUM does not change (DATUM: Beyond the compression of a reloader) the term is still thrown around like it is a line, for me the datum is the surface of a set-up table, when referring to chambers and cases the datum is not a line it is a round hole, a straight round hole perpendicular to the surface, for me the surface becomes the line.

When fire forming 30/.06 cases in an Ackley chamber the short neck of the parent case hits the neck/shoulder juncture because it is shorter, when the case is chambered the neck is sized in the Ackley chamber, some call this head spacing, when chambered and fired the case body is blown out and the shoulder of the case is blown forward to form the case.

Then there is the ACKLEY chamber, if a new barrel is used most can act like they know what they are doing but when a 30/06 chamber is reamed to an Ackley improved chamber all of the old chamber can not be cleaned up because of the long neck Ackley and short neck 30/06, if the barrel is not moved back part of the old chamber will remain, or the reamer can be advanced to remove the original shoulder, but, there goes the datum and the use of the go, no and beyond gage, as I have said, I do not use the go-gage, I do not shoot gages I shoot ammo and measure chamber length from the datum/shoulder back to the face of the bolt in thousands. Or the 30 Gibbs could be a consideration, the neck on the 30 Gibbs is 'about' .217 long, again problems are presented when cutting a 30 Gibbs chamber in barrel that was chambered in 30/06, more likely than not there will be those that talk about it but never get around to doing it

In my opinion when going up from a 30/06 to a chamber that gives more speed with heavier bullets, the 308 Norma is a better choice than the Ackley, then there again is the 30 Gibbs

The 257 Roberts when chambered in the Ackley version lives up to 'improvement' as does the 7mm57 when chambered to the Ackley Improved chamber, Not all 30/40 Krags were single lug bolt designs, some were made by manufactures that knew how to install a bolt with two lugs, for those rifles the Ackley Improved chamber makes for a big improvement in performance as does the 303 British if the design does not have rear locking lugs like the Enfield, the P 14 when chambered to the Ackley Improved chamber is another chamber that benefits from the genius of Ackley.

F. Guffey

August 6, 2010, 11:25 AM


Look for cases that are longer from the shoulder back to the head of the case when forming, as in forming first then fire instead of firing to form. The 280 Remington shoulder is ahead of the 30/06 by .051 thousands, for the few reloaders that can keep up with case length the 280 can be necked up and sized for a 30/06 chamber that has as much as .051 thousands head space, so there is no reason for firing a case in a chamber that 'has head space issues' as most know I have an Eddystone M 1917 with .016 head space when compared to 'THE BOOK', I adjust my die to the shell holder in the press to size 280 cases to 30/06 that have an additional .014 thousands between the shoulder and head of the case AND add the additional .014 to the length of the COL and CL (case overasll lenth and case length.

There is a link above to Cylinder Brass by R-P, the case have no shoulder, just a straight wall, this stuff came along too late for me but the case is 2.650 long, great stuff. When forming 30 Gibbs I necked 30/06 cases up to 338 and 35 Whelen to create a shoulder to head space on, I form and fire once, I do not use cereal, I laod-um up, some descrobe methods I have used as 'some scary stuff', they always leave out the time factor.

And I have purchased once fired cases from ranges when I can find them for .08 cents each, $8.00 dollars per hundred + 10 extra, This may not be a bargin to some but I measure the cases for length from the shoulder back to the head of the case, my favorite cases are the ones that have been fired in long chambers, all I have to do is know where the shoulder is and adjust my die to the shell holder in the press, a tough press, one that does not flex or yield or distart. Yes moving the shoulder forward is the tough and expensive part, I try to start with a case with the shoulder forward, then move it back.

F. Guffey

August 6, 2010, 11:54 AM
Datotsin, the forming of a case to off set the effect the chamber will have on a case when fired is not complicated, the case when fired will conform to the chamber and will make a mirror copy, there are some that can not accomplish this method because they have 'spring back''.

When forming a case in a chamber without knowing the head space is a bad habit and done by those that can not determine head space first, but for new unfired brass there is forgiveness for error, 8mm57 ammo has been fired in 8/06 chambers, that is .127 thousands head space, the results? The 8mm57 case when ejected came out looking like a 8/06 case with a short neck. Confusion at big time matches caused some shooters to chamber 308 Winchester cases in a 30/06 chamber, when ejected the 308 W cases looked like straight wall case without a shoulder, again Hatcher moved the shoulder forward to create head space and in doing so he created the Hatcher 30/06 modified chamber.

I determine head space first then adjust the die to the shell holder, size then fire, others fire first then measure the effect the chamber had on the case, then they go into the twilight zone as what do they do with the information after it is determined the chamber allowed the case to stretch, one of those terms that reloaders do not have in their vocabulary TRANSFER.

I apply the leaver policy for fired cases, once the shoulder moves out to fit the chamber, I leaver out, I use the companion tool to the press the feeler gage because I know the deck height of the shell holder, the length of the sizer die from the shoulder to the bottom of the die and my presses are not springy, my presses do not yield before the case gets sized, even then if a case whips my press? I can measure the amount of the case that does not get sized in thousands.

F. Guffey

August 6, 2010, 02:16 PM
In short, powder capacity. When lived in AK many winters ago I had a .35 Gibbs (also based on 06 case) it certainly had the powder capacity. When zeroing my shoulder gave proof positive as to recoil.

August 7, 2010, 09:57 AM
thank you for the responses fellas... i don't think i'll do the ackley thing, but does explain how ackley works and merely having too much headspace isn't the same thing.

August 7, 2010, 02:52 PM
One thing to remember is, while Ackley, and a lot of other wildcatters were developing those higher velocity wildcats, pressure measuring was something they only did to their car tires.

Once modern pressure testing equipment became widely used, it was found a lot of those wild claims of super performance were not so super after all when held to the same safe & sane pressures used by the factorys.


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