AI Chamber

[Hammerhead]

Hey Guys,
I have a reamer from PTG and a go gauge for the same.
It is a 22-6mm. I have done one chamber with it with no problems on fire forming brass.
My question is I have read on here before that you need to leave the chamber a little short for a crush on the brass for fire forming, how much?
I have done a search and can't seem to find it here.
Anyone care to help me out on this one?

Thanks,
Tim

 

[eskimo1]

Never done a AI but a couple of ppc chambers. What i do is that i use a case for a gauge and cut the chamber until i get a crush fit on the case. A couple of thousands crush should do it. You feel the crush when closing the bolt on the action.

 

[WayneShaw]

An AI go-gauge is .004' shorter than the parent case go-gauge, IIRC.

 

[Hammerhead]

WayneShaw,
I did use a AI go gauge and the bolt was stiff to close on the first chamber.
Am I good to go?

Tim

 

[Preacher]

It shouldn't take a lot of pressure to close the bolt on a ackley chamber, but you should be able to feel it with the firing pin assembly out of the bolt.
You need to have that contact, but no more than that...

 

[Ackman]

You want to feel pressure when the bolt closes. And really, on these things the amount of crush is up to you. Measure headspace on the brass you'll be using if you have a way to do it. You can make a gauge using about a 1' piece of barrel cut-off..... run the reamer in just enough for a neck and shoulder. Cut a lengthwise pie shaped window and you can also use it to see brass neck length in relation to the chamber.

As an example with some .243 brass. I measured a couple dozen each of Win., PMC, and Lapua cases.....from casehead/boltface to neck-shoulder junction. Each headstamp varied by .004'-.005' shortest case to longest. And the shortest of one headstamp was .009' less than the longest of another headstamp. I don't know where those measurements fall in relation to a go-gauge, but that's a pretty big gap. My .243AI chambers are set with up to .015' crush on Win. brass and it works beautifully.

The number that gets thrown around is .004' crush. But if that's off a headspace gauge and since brass can vary quite a bit, it could easily not be enough. You can use however much you want. It's between you and the gun. Crush is just the neck/shoulder radius of a new unformed case hitting that corner of the chamber. With even .015' crush - which sounds like a lot - bolt pressure isn't real heavy. Keep lube on the lugs and things will be fine.

 

[tightneck]

WayneShaw,
I did use a AI go gauge and the bolt was stiff to close on the first chamber.
Am I good to go?

Tim

yes

 

[WayneShaw]

Ackman, you are correct in saying the crush is a variable. Brass thickness can directly play a part in the bolt feel. The -.004' measurement is an average I suppose that works most of the time.

But .004' isn't much with a brass case. I think the whole idea of Ackey's was to be able to run his reamer into an existing chamber and blow out a case. If that is true, you will never get a -.004' gauge to work. The best you can do is get the shoulder/neck junction 'point' as one, no more, no less.

To use a -.004' go gauge on an existing chamber, you need to set the shoulder back some, re-time the barrel if it has sights, etc. That wasn't the original intent.

There are ways around all this .004' thing. What you really need is a way to keep the brass fully seated against the boltface while fireforming. You can jam a bullet real hard and fire it,with a lot of neck tension), or you could expand the neck up one caliber, then resize it back only enough to get your crush bolt close back.

That .004' long brass will never give any trouble, as long as you properly adjust your sizing die afterward. After all, the difference between a go and no-go is what .004 or .006?

 

[Hammerhead]

Thanks for your help guys.
I believe I am squared away now. It seems the more I read the less I know.
I got this one licked though.

Thanks again,
Tim

 

[Rust]

Old P.O. was a pretty smart fella. There were a couple of ideas behind the headspace being .004' shorter than the parent chamber in what is considered the final version of an AI chamber. He did a lot of experimenting with the amount of taper in a case and the shoulder angle. Some variants had the taper taken out but retained the original shoulder angle, some kept the original taper but had the shoulder angle changed to a sharper angle and not always 40 degrees either.

The commonly heard reason for the .004' shorter headspace is that it locks the case to be fireformed into the chamber at the neck shoulder junction. A lot of brass is a little shorter and will not lock in.

Another reason, seldom heard is that the slightly shorter headspace keeps the brass from being stretched too much above the web of the case which would be bad for a couple of reasongs. Even if the case is not locked in at the neck/shoulder junction. Couple of reasons why that is a good thing.

1st, the brass at the case head has not been annealed as has the area at the nack and shoulder thus stretching the brass above the web is more damaging to the brass as the harder brass is not as mallable. Slightly shorter chamber, less possibility of metal fatigue due to excesive stretching. Embrittlement is not good.

2nd, along with that excessive stretching might thin the metal of the case which leads to case head seperation. I know I've seen those shiny rings right above the web before, indicative if the case getting pretty thing right there. Sectioned a couple too and they were thin.

3rd, and I don't know for sure if it has any real effect, the idea that it will help keep the neck from shortening as much when the shoulder blows out and tries to pull some metal back from the neck and turn it into shoulder. 22-250 seeems to be more of an issue than a 6mm AI simply because the 6mm has a lot more neck to start with. And the 22-250 has a lot of taper in the parent case too.

At any rate, if the .004' shorter dimension helps keep the area around the web of the case from becoming brittle or thinning too much, sounds like a winner to me and since no one I know of can prove otherwise I'll stick with it.