What is causing thick necks?

[tfbit]

Like dellet said, can't be. Bullet should be 0.308" plus 2x 0.010" would be 0.328". 0.334" is the max SAAMI dimension. You have to have a neck thickness over 0.010". 0.034" mouth diameter added to a tight chamber and you have problems.

Where the bullet meets the brass is .334".

[tallburnedmidget]

Tallburned: Correct, it does pass without issue before seating bullets.

Dellet: 208 AMAX 10.3gr IMR 4198, brass length 1.358-.360 OAL: 2.243
I ran the brass as recieved, no sizing on my end
Base of the neck measures .330


I adjusted my seating die a little further down and it seemed to correct the issue for the most part, had 3 cases out of a hundred that swelled after the neck for some reason but adjusting it seemed to fix the issue.

Yeah play with the seating die a bit. In one of my situations I had to adjust my die because the built in roll/taper crimp was being engaged and that ridiculous feature for the 300BLK die ALWAYS knocked my necks/rounds out of spec.

Keep playing with it and checking with your gauge and I think you will get it dialed in. This is exactly why the Sheridan Gauge has been so helpful for the 300BLK. The 300BLK seems to be more easily susceptible to knocking out of spec compared to other calibers, but once you sort it out you won't have any issues from that point on.

It can just takes a little tinkering at first Best of luck!

[2manyToys]

I've seen poor internal chamfers cause random issue like this. Have you taken any of the offending cartridges apart to look for issues? You might find scoring in the jackets that relate rough areas in the brass.

[Ripcord]

I did take a couple out and the bullet did have some scoring on it, perhaps they do just need to be chamfered a bit more.

[dellet]

I did take a couple out and the bullet did have some scoring on it, perhaps they do just need to be chamfered a bit more.

Inside then outside.

Not doing that well, will give varying contact points for the roll crimp as will varying lengths but it looks like you have length covered.

If there is an outside burr you can generally feel it with a thumbnail on a loaded round. If you just touch the edge with a file, you can generally knock it off and have the round chamber/gauge.