Variations in Fired Brass Neck Diameter?

[dakor]

I will try make this as simple as I can. The rifle is a 7mm stw and the reamer has a .317 neck diameter. When firing Winchester brass the cases measure .315 on the outside something you would expect. When using a Bertram brass the fired cases measures .310?????

I am thinking the Bertram might need to be annealed? Your thoughts?

 

[Pwc]

What is the difference in neck wall thickness? ID vs OD?

 

[dakor]

The Winchester thickness is .015 the Bertram is .010.

 

[D-4297]

Sounds like the Win brass expanded to neck chamber diameter , as it should and re-coiled back a almost normal .002 . If the Bertram brass went down to .010 , you have to ask yourself what the "pre-loaded" outside diameter was , and how much interference fit it had before inserting the round . Seems like a lot more than normal spring-back on the Bertram for a .010 case wall . I would think that the thinner wall brass would expand normally , but not rebound that severely . Might be time to anneal them . I anneal after every 2nd firing just to keep stuff consistent .

 

[cw308]

Bullet OD .284 add neck thickness for both 1 is .304 2 is 314 chamber .317 if my bullet measurement are correct , the Bertram case has a lot of room from .304 to .317 how much can brass expand , does it make it to the .317. How did it shoot ?

 

[dakor]

It shoots 1/2 or less at 100 yards but opens up to 3/4 to 1 moa or more at 200 and beyond. The flyers is what is causing the group to go large and they are random it can be the first, second, or third shot going out of the group. Not consistent at all. The Bertram measures .307 with a 180 Berger VLD seated.

Just seems odd the brass is not blowing out close to neck diameter and I asked him how hard it is to put a bullet by hand in a non sized fired case. He said it was tight when he tried it but could push it down the case mouth.

More and more I thinks its the brass causing the issue he is having.

 

[robster7]

Would cryogenic treatment of the brass provide any metallurgical benefits in this situation? It would be interesting to see if the cases maintained their dimensions and tension consistency more uniformly after cryo treatment???

 

[dakor]

Not sure on cryo treatment but it was the brass even after anealing it still had issues. He switched to Remington brass and the cases blow out nice and the rifle is shooting 1.25 inch groups at 420 yards.

He spoke with Bertram and they did admit they have had some bad lots of brass. They are refunding his money anyway.