Fire forming AI loads (difficult to chamber)

[Lightfoote]

I’ve got my first AI rifle put together (6mmAI). I’ve got a bunch of brand new Nosler 257 Robert’s brass to neck down. First I chambered new rem 6mm brass and the bolt will close with resistance. Then necked down a 257 brass with 6mm sizing die. Neck down went fine but brass won’t chamber. Checked bump and it’s the same as the 6mm rem brass. Check other dimensions and came up within .001 of rem 6mm brass. Then forced the bolt closed on the necked down 257 brass and got a shiny ring around the neck shoulder junction. Probably not quite sizing the junction all the way. Is this a typical problem? What’s the fix?

 

[Bill Norris]

I’ve got my first AI rifle put together (6mmAI). I’ve got a bunch of brand new Nosler 257 Robert’s brass to neck down. First I chambered new rem 6mm brass and the bolt will close with resistance. Then necked down a 257 brass with 6mm sizing die. Neck down went fine but brass won’t chamber. Checked bump and it’s the same as the 6mm rem brass. Check other dimensions and came up within .001 of rem 6mm brass. Then forced the bolt closed on the necked down 257 brass and got a shiny ring around the neck shoulder junction. Probably not quite sizing the junction all the way. Is this a typical problem? What’s the fix?

With my experience with the AI chamberings the case will fit fairly snug when chambering the parent round but not to a point of not chambering. I have not had to bump shoulders on any parent case so far.

 

[Dave Berg]

My 'smith, may he rest in peace, never cut an AI chamber that I didn't have to use brute force to close the bolt on. He called it a little persuasion. If I bitched enough he'd run the reamer in a little more with a hand tool but there were always some light ring marks until the first firing. BTW, the 6mm Remington Imp he built for me was my favorite groundhog rifle ever.

 

[Lightfoote]

I think I got it figured out. I set up two presses one with the straight 6rem die and one with the 6AI die. Neck down with the 6rem then run the brass not quite all the way into the 6AI die. Did the trick, I can close the bolt with slight resistance.

 

[358WCF]

You may know all of this, but maybe not? The "go" headspace gauge becomes the "no-go" gauge on Ackley chambers. I believe the "go" is .004" shorter. Necking down to 6mm from 257 is a good move to make proper headspace when fireforming as long as your neck diameter isn't too snug. Are you sizing the 257 brass with a standard 6mm die or the Ackley die? I've telescoped & collapsed shoulders on 22-250 brass using the Ackley die rather than the standard die because the upper case wasn't supported.

Just size until they chamber with a little feel. Brute force isn't a good thing. The shoulder bump gauge wont tell you much in this instance. The spot where you're getting the shiny spot... neck shoulder junction... is the only place there will be contact until the shoulders are blown out. I would size your brass a tad more until the shiny spot isn't there & the bolt can be closed with some feel, but without an act of congress. Keep the back of your locking lugs lubed while doing this too.

Fireforming loads with powders on the faster side at or near max for the parent case (6mm in your case) seem to blow the shoulders out best for me. A good snap is better than a wimpy load twice. I usually start a little below max & increase as necessary for nice sharp shoulders.

Here's a pic I shamelessly stole from somewhere on the internet showing where the case/chamber meet... beats a 1000 words... Good luck.

 

[boltfluter]

A snug fit is good for fireforming, but keep the lugs well lube with a little grease. Galling is not a good thing. Carry on.
Paul