If you don't know what flat primers mean, that's not good. It is fixable through education, however, and that's a good thing. Glad to see you're OK. One question that hasn't been asked, and may be your problem, is what are you trimming your brass to? Are you trimming to JD Jones' length of 1.4"? If you have a 1.355" chamber (nominal) you are going to have the problems you are describing. Also, what is the jam length of your rounds? Perhaps you are jamming your rounds into the lands. Take a look at the loaded rounds upon extraction. Do you see rifling marks at the ogive?

Respectfully,

Kevin

I agree with N310toN170's logic--neck tension causing the sticking of unfired rounds and causing higher than normal pressure upon firing causing the flattened primers and stuck cases. But definitely check your case length at 1.355" or less.

Dykem up one of your rounds and carefully place it into the chamber. Release the bolt so it slams shut. Remove the round and see were the Dykem has been rubbed off. This will tell you were your brass is sticking. You can also use a black marker to color the round if you don't have Dykem.

Brass is 1.358"

There are a number of long scratches that go around the diameter of the shell case (say from 12 o'clock to 6 o'clock, starting just past the shoulder and ending about .12" before the rim - about 6 of them evenly spaced apart.

No rifling marks at the ogive.

Any suggestions on how to measure jam length?

What does your case neck measure on a loaded round?

What does the neck measure on a spent case fired in the 8"?

What does the neck measure on a spent case fired in the 16"?

What is the origin of both barrels?

At work now so can't measure, but will later. Are we talking case neck diameter or case neck length measured from the shoulder to the end where the bullet went in?

Origin of both barrels was from a member here who makes custom gas blocks and barrels for the .300 - can't recall his name/handle off the top of my head but I think he does a lot of them.

Knowing the fired case neck OD will allow us to know how tight your chamber is in that area. Typically, that measurement will be .001" larger than the fired case neck OD. As far as the scratches go; run a cotton patch into the chamber area and check for burrs. It couldn't hurt to polish out that chamber area either; stuck cases or not. A little JB non-embedding bore paste on a spinning case will do the job nicely. Hornady (formerly Stoney Point) makes a tool for checking the jam length of a barrel with a given bullet. Here's a link:

http://www.hornady.com/store/Lock-N-...Straight-1Each

There is a lot of knowledge here; feel free to tap into it as much as you like.

Kevin

Case neck diameter. The reason I'm asking for multiple measurements is because comparing the loaded neck diameter to the fired neck diameter will give you an idea of how much the neck is expanding to release the bullet. I asked for both the 8" and the 16" to see if the 16" is actually tighter than the 8" that isn't giving you problems.

You are loading to too high a pressure. 18.5 grains of H110 with a Nosler 125 to that OAL is not reasonable.

Case neck OD on loaded round = .330

Neck measures OD on a spent case fired in 8" = .333

Neck measures OD on a spent case fired in 16" = .336-.339

Case length on loaded round (both super and sub) = 1.363-1.366

Case length on fired round from both = from 1.355-1.385