Reply with quote #9
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.
Reply with quote #10
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.