Thread: ? Head Space

Head Space question, have a 7mm08 AI barrel which I aim to put onto a 308 rifle, once it’s not shooting.
Would a standard 7mm 08, go/no go gauge be ok to set up the head space?

Doesn't a go gauge simply become the no-go gauge for the AI case?

Originally Posted by Pommy

Doesn't a go gauge simply become the no-go gauge for the AI case?

If that were true it would not be possible to fireform standard brass in the tighter chamber.My theory would be use standard gauges to set up.
Consult your gunsmith when refitting would be my solution.

Good question,

As you should be able to fireform factory 7/08 in the AI I would say headspace should be the same.
a google of the SAAMI specs should tell you.

Was just wondering I bought the gauges, I could do it, have a heap of fireformed brass came with it. Best practice to get a gun smith to do & true all at the same time.

AI cases are shorter to the base of the neck and longer to the point of the shoulder plus the shoulder is much steeper. I very much doubt if the standard head space gauges can be used due to the much steeper shoulder.
GPM.
Dug around a bit and it seems that the headspace gauges have a truncated cone shaped front to match the shoulder taper of the case. This is likely to mean that you can't use a standard gauge to set headspace in an AI chamber. Others with direct experience may have more to say on this.
Grandpamac.

@Tedz50, have you messed around with AI cases before? Because you're supposed to have a crush fit when fireforming. Otherwise you run the risk of case head separations, because the shoulder is unsupported.

Googled it... https://ackleyimproved.com/headspace...ed-cartridges/

So the proper headspace gages for a rimless Ackley Improved chamber are a “go” gauge made for the specific Ackley caliber, and a “go” gauge from the factory caliber becomes the “no-go”. This works because the Ackley gauge is .004” shorter than the standard “go” gauge for the given caliber. Because such a small portion of the shoulder will actually be in contact with the shorter chamber the resistance one might expect is almost nonexistent.

Originally Posted by Pommy

@Tedz50, have you messed around with AI cases before? Because you're supposed to have a crush fit when fireforming. Otherwise you run the risk of case head separations, because the shoulder is unsupported.

Googled it... https://ackleyimproved.com/headspace...ed-cartridges/

@Pommy No have never fired an AI cartridge but am always interested in Questions such as these hence my question and suggestion that the gunsmith who did the job would know.I too would delve deeper if it was my gun @flock have some 708 cases if you want them.

Ok thanks All, beyond my abilities, will take it to a Gun smith & get all done properly. Could be years away, 7mm projectiles accessability hopefully improved by then.

In theory it is possible to set up a 308 with a 7-08 head space gauge , not the other way around , as the two rounds are the same except from some obvious differences . Head space is measured from the base of a cartridge to the point at which it seals the chamber ,so the only difference being calibre which puts the neck a little further out and if the gauge can chamber then in theory it will work as long as practice does not rear its ugly head .
So in the end it is a double headed question of theory versus practice and using the correct gauge eliminates the dilemma.

I thought the shoulder/ neck junction length were the same on both a std and AI?

Originally Posted by 6x47

I thought the shoulder/ neck junction length were the same on both a std and AI?

Head space is not just one measurement of X to Y , for most applications . Usually it is on the whole of the shoulder , from where the body ends to where the neck starts for rimless rounds .Head space is about more than just pulling the trigger and rifle goes bang , it is about fit/tolerances etc .
As an example at the beginning of the WW1 they had a problem of rounds not fitting easily in rifles because of a lack of tolerances/ quality control , so in effect when you head space a round you are only setting the rifle up to accept rounds that are of X size and no more than X in dimension so that A they fit and B the case will seal the chamber for all rifles . So in effect a Rifle head space is so that you can pick up any case/live round and if you load it the rifle it will fire it safely because it is within the in the accepted range of dimensions .

Originally Posted by 6x47

I thought the shoulder/ neck junction length were the same on both a std and AI?

For those with QuickLOAD, the dimensioned drawings supplied may be used to compare; say .257Roberts/.257AI, 243Win/.243AI, 30-06spr/30-06AI and so on. This comparison will find that officially the improved cases are all 0.004" shorter at the shoulder-neck junction, plus or minus maybe half a thousandths depending on the particular pair of designs.
There will be plenty of barrels out there in improved chamberings that will have been fitted to "feel right" on a sample of a few parent cases supplied at the time. As Moa Hunter has said above, within reason it doesn't matter so much, as long as the full length or body die being used is set up to keep the cases snug in that particular chamber. I have had one improved chamber where the initial crush fit was a set up a little enthusiastically, and I needed to grind a bit off the base of the die to make it usable. With some variation it just means your improved or wildcat chambering is a little more wild. A good rule for long case (and personnel) life with AIs is probably to not mix rounds between rifles unless they are a standardised design – of which only the .280AI comes to mind.