Headspace and datum line

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adosland

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I'm planning to build a 257 roberts Ackley improved hunting rifle. Before I get to far down a road of buying build parts I was hoping for some advice.
I managed to find a new 257ai finishing reamer to chamber a barrel. It came with go/no go gauges marked 7x57 mauser.
I know 7x57 is the parent case to 257 roberts but will these gauges work on 257 roberts ai? Just trying to confirm because the shoulders are different. Wouldn't this affect the datum line ?
Thanks
 
Ream the chamber until it will just close on the GO gauge. You should be able to feel the gauge. Minimal chamber. It is a good thing if you can feel a factory round when closing the bolt.
Then .257 factory cartridges will be supported for first firing.
 
I think that you are correct that reaming to AI will move the datum line forward - but I think when a parent case is chambered in an AI chamber, the cartridge is "headspaced" at the junction of neck and shoulder - not at the datum line. So that is an interesting progression - from a parent to a parent to an AI case - 7x57 and 257 Roberts (and 6mm Remington) usually share the same shoulder angle and same datum line - so same GO gauge used. 257 Roberts AI uses "flatter" shoulder angle - a larger numerical angle at shoulder and smaller numerical angle at neck/shoulder junction - (and usually the front part of case is also blown out (makes less body taper), when standard case is fired). But the GO gauge should be ground with the parent case shoulder angle AND datum - so the junction of neck and shoulder should remain at same place - although the "datum" has moved forward. So parent GO gauge can still be used - assuming reamer and gauge both correctly ground to spec - except the gauge will be "touching" at the neck/shoulder junction in the AI chamber, not on the "datum"
 
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I know you can fire a regular case in AI chambered rifles but where they headspace may be different. I'd phone the reamer manufacturer and ask as no doubt you're not the first person to inquire.
 
Ackley Improved chambers are headspaced 4 thou smaller than the original cartridge. It is a different headspace gauge. An original cartridge should be firmly felt when closing the bolt in an AI chamber.
 
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