Chamber cast

[Bill Daniel]

I made a chamber cast today of my Criterion 6.5 mm Creedmoor AR 10 barrel. All the measurements looked good except the diameter near the base that was more elliptical than cylindrical. Is this how an AR chamber is made, or an artifact of my casting, or what? Measurements at the base rotating in the caliper jaws run from 0.463”- 0.469”.
Thanks,
Bill

 

[T. O'Heir]

I'd re-do the casting just to be sure. Cerosafe is reusable. An oval chamber isn't good. Indicates the chamber reamer was wobbling. 6 thou sounds like a lot to me.
You have access to an inside micrometer? Don't buy one. They're really expensive.
You can measure a fired case at the base.
Maybe an idea to give Criterion a call too.
https://criterionbarrels.com/contact/

 

[Bart B.]

Out of round chambers that much happen when low quality tools and techniques are used.

Ask Criterion about that.

 

[Bill Daniel]

Thanks fellows. The out of cylinder area is right above the extractor groove when compared to a cartridge. Just distal to this it is cylindrical at 0.470”. I will recast it this week end and if truly out of round will give Criterion a call. I have checked the same area on fired rounds checking for over pressure signs and found no significant expansion.
All the best,
Bill

 

[FrankenMauser]

I'd lean toward a casting defect.
But I've seen out of round chambers, too. It's perfectly plausible.

I have a Ruger 77 Mk II .270 Win with a "whale bellied" chamber. It's so far out of round that you can't rechamber fired cases without 'clocking' them correctly. But, it shoots and doesn't have extraction issues. No point in messing with it...

 

[4V50 Gary]

could be bad tooling

 

[SGW Gunsmith]

When doing chamber casts I use some stuff called "Facsimile". When I worked for a supplier to the military for obscure parts that had no prints available, I needed to make internal castings of parts so that the dimensions could be replicated.
I still use this product in my shop when I find screws missing on older .22 rimfires that I need to restore. Only way I can get an accurate thread pitch and diameter so I can make one.

 

[Bill Daniel]

Repeat cast measures well. Chamber neck 0.296” Shoulder 0.463” Above extractor groove 0.471” Leade 0.2” Groove 0.264” Lands 0.261”
Thanks to all,
Bill