300 Win Mag Brass issue

[Old Dude]

I am working up loads for my new rifle. Rem 700, 26 inch barrel, AI chassis. I am using Hornady brass, H1000, CCI mag primers, and Hornady 208 bullets. I started with new brass and am using RCBS dies. I started at 72 grains and worked my loads up in 1/2 grain. 74.5 and 75 held the tightest groups. 76.5 had a hard extraction so I stopped there. I took a new case and a slip fit bullet into the rifle and slowly closed the bolt. I measured 3.672. I set the die to give me 3.662 COAL. I single feed the rounds because they wont fit in my AI mag. No flattened primers. Heres the issue. New brass measured 2.612. Now the once fired brass measures 2.602. I expected a bit of growth. This is my first time reloading a belted round. Neck expanded from .340 to .342. Brass looks great. I'm going shooting this weekend and wanted some input.
Thanks
Bob

 

[Old Dude]

I did a dry erase on a once fired. It looks like I'm seating on the belt and the bend of the shoulder.

 

[Old Dude]

Found my problem Had new and once fired mixed up. everything checks out now.

 

[Mendoza0311]

Attention to detail Just try to be careful not to mix up your new brass with fired. I’m also loading 300win mag 76.8 H1000 Fed Mag Primers and 180 Berger’s they’re are ridiculously accurate. If you want to load to the mag length. You should do a jump test before just loading the bullet out so far. Have you tried to measure to see how far you are off the lands?

 

[Old Dude]

I’m .015 off. What I did was load an empty new shell with a barely seated bullet. I slowly closed the bolt then measured it. I set my press to give me a .015 shorter length. thats from the tip to the base. Just received my comparator set. So I can now measure shoulder set back.

 

[culater]

Need to add this to your reloading practice. I have one on the way.