New to loading 300 blackout, and rounds won't fit in case gauge

[GravelRider]

Hello all, I'm having some trouble with reloading this round.

I'm new to reloading for the 300 blackout, but not new to reloading. I've been reloading for nearly 20 years. The 300 blackout is the first round in which I've ventured into case forming. And it is giving me a run for my money so far.

I'm loading on a Dillon XL650 and Redding T7 turret press. I used the Dillon to decap/form and prime. I then have been hand measuring the loads and using the Redding to seat the bullet and crimp. Eventually I will be using the Dillon exclusively to load once I get things ironed out. I am using Hornady Custom grade dies and a Lee factory crimp die.

Recipe: Berry's Bullets 200 gr .308" plated SP bullets over 11.5 gr CFE BLK. Obviously, I'm loading this for subsonic.

I started with mixed once-fired 556 brass. I dry tumbled in corncob media overnight for a high shine. I sorted the brass according to the good/bad list on this site. I built a jig, and cut down the brass to around 1.370."

I then deprimed and sized/formed the brass.

Then I trimmed the brass to 1.358."

I then dry tumbled again in corncob media, and ensured there was no media stuck in the primer pockets afterward.

Then I checked each piece of brass in a Sheridan slotted case gauge. They all fit with no issues (well, I had to toss about five cases that wouldn't fit out of about at thousand I had formed).

Then I finished loading the rounds, with on average OAL of 2.239."

I lastly test fitted each finished round in the case gauge, and only 2 out of 20 fit!

It appears that the neck is too wide (see picture below), but the neck diameter measures 0.332." This is well within specs.



I loaded up five of them (all that wouldn't fit in the case gauge properly) into a magazine and chambered and extracted them with the charging handle, and they all chambered properly, though extraction required a healthy pull on the charging handle, so clearly they're not fitting into the chamber as they should.

The dimensions are all where they should be, yet they're not fitting in the case gauge. I'm quite stumped.

I'm hoping you guys may be able to point me in the right direction.

[GravelRider]

And I forgot to mention that I did not anneal the brass at any time during the process. I have since purchased the Annealease annealer, but didn't use it on these, since I purchased it after I already formed the brass.

[dpete]

What headstamp brass did you end up using? I know you said it was sorted using the good/bad list but there are some that are more gooder than others.
WCC, and Lake City are my go to cases now, but I've used RP, PMC, FC, and Winchester in the past with no problems.

[dellet]

Most likely you’re loaded too long. There are a couple Ways to verify. But start with whose data did you use to come with the length you used?

There may be rifling marks on the bullets your chambered.

You should be able to see in the gauge where if it touching.

Measure the neck diameter, needs to be less Than .334”, those bullets are .309” diameter and it can be a problem sometimes.

You can drop a bullet in the gauge and then set a case on it. Measure bullet tip to case base. Then Measure the amount of case above the deck of the gauge and subtract that from overall length, that will give you max length of a loaded cartridge. Back off that .010” minimum.

[30plinker]

You can drop a bullet in the gauge and then set a case on it. Measure bullet tip to case base. Then Measure the amount of case above the deck of the gauge and subtract that from overall length, that will give you max length of a loaded cartridge. Back off that .010” minimum.

And if that number is shorter than what you are currently loading (it should be if this is your issue) be sure you are off from the max powder level (more bullet in case == higher chamber pressure at start)