That's the .223 PawPaw Improved. It's a new cartridge I designed today. It's an interesting design, but I'm not sure I can get anyone to cut a chamber for it, and I don't know how I'd extract the brass.
You'll notice that weird ring at the shoulder, that's the main design flaw.
I was at the bench today, reloading .223 Remington, using the Hornady V-Max bullet. After doing everything right, up to the point of bullet seating, I wasn't paying attention and screwed the seating die into the press. When I seated the first bullet, I felt something odd and withdrew the case. I figured maybe I had a piece of bad brass, so I put another piece of brass in the press. Got the same result.
As it turns out, I had the seating die too far down into the press. The crimp shoulder was grabbing the neck of the cartridge, couldn't crimp it, and was collapsing the brass at the shoulder.
Yep, even PawPaw screws up once in a while. I properly adjusted the die and the ammo started coming out perfectly. Lesson: Pay attention while reloading. Little details are important.
Want a close-up?