Issues loading 300aac. Round won’t headspace after bullet seated.

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Shooter14854
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Issues loading 300aac. Round won’t headspace after bullet seated.

Post by Shooter14854 »

Hey everybody I’m hoping somebody here could give me some guidance. I recently got the tools to convert .223/556 brass into 300aac. I’m using the RCBS short base dye for sizing. Trimming the case to 1.360. I’m loading Hornady 150gr HPBT. I measured the neck thickness (only using brass with .013 or lower neck thickness) of every piece of brass after trimming. All of my converted brass is seating perfectly in my Hornady headspace gauge. However when I seat the bullet into the case (2.090 COL) the completed rounds will not sit flush in my headspace gauge. About 95% of them stick out about 1/8th inch. I’ve tried resizing them with the RCBS dye again, a Lee dye and checked if it sat after doing a taper crimp. The round seem to seat fine in my rifle but I havn’t tired firing them yet. I didn’t have any rounds fail to fully seat in the rifle or get stuck.

Additionally, I tried running once fired factory 300 blackout brass through the same dye with the same process as my converted brass. All of the factory brass seats fine in the headspace gauge after the bullet has been seated. I’ve compared the measurements on my converted rounds to my rounds using factory brass and they are virtually identical. I have no idea what to do and am completely out of ideas. Any help would be very appreciated! Thank you all!
imashooter2
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Re: Issues loading 300aac. Round won’t headspace after bullet seated.

Post by imashooter2 »

when you seat the bullet, it expands the neck. Sounds like some of your brass is just a bit too thick for your case gauge.
exexex
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Re: Issues loading 300aac. Round won’t headspace after bullet seated.

Post by exexex »

The bullet seating seems to be where the problem occurs. I can't imagine it being the Hornady bullets being out of spec. As imashooter2 says, it might be thickness of the brass. Is it 556/military headstamped brass? If so, that might be the problem as I've heard that military brass tends to be thicker internally (same outside dimensions, though). Have you confirmed trying this with 223-headstamped brass?

Only other thing I'd ask is if you see any deformation of the seated bullet or case, like around the neck.
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dellet
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Re: Issues loading 300aac. Round won’t headspace after bullet seated.

Post by dellet »

There are a few things to check, first as mentioned, loaded neck diameter. Check it at the very top and bottom. The thickest brass will be at the bottom. At the mouth, if you did not do a good deburr and chamfer after trimming it can hang up. Sometimes you can feel a burr with your finger. If the mouth has a burr or slight flare, sometimes a quick hit with a file will correct the problem.

Loaded length could be right on the edge of too long, and bullet variation could be the issuer. You would need to check the length form cartridge base to bullet ogive to find this. Or just seat the bulges a bit deeper. Sticking out 1/8” is more than you would expect for this type of problem, but it’s good to think about.

Runout. The bullet not seated straight. You might be able to roll them on the table and see the wobble.

Highly compressed load can deform the brass, and can cause the above runout.

If they chamber in your rifle, they are likely good to go. The COL seems short for bullet, but if it came from a trusted source, it should be fine. I have found enough bad data over the years that I always confirm that published data will actually fit the chamber. That’s part of what the gauge is possibly warning you to check.
300 Blackout, not just for sub-sonics.
ironhead7544
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Re: Issues loading 300aac. Round won’t headspace after bullet seated.

Post by ironhead7544 »

Had that same problem. Got the Hornady seating die with the sliding sleeve. That fixed the problem. That die seats the bullet straight.
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