Previous 2 posts, show firing pin issues and Pressure signs.
There are enough replies, with good information, and I hope you are now on the right track.
I'll just add a few of my experiences.
> A CZ with a loose fitting firing pin, or oversize firing pin hole, that would make primers look like that, pressure didn't matter.
> A Ruger M77 II Lightweight, didn't like starting loads, needed a SR Mag primer, CCI 450. Increasing the charge changed that. But I still start with a mag primer.
> Several other 223's that make me believe they like top(ish) loads. My theory, long throats and chambers cut to lower pressure for military ammo.
> The Hornady Lock-N-Load headspace Comparator is a must have tool, as are the Redding Competition shell holder sets. However, I seldom bother with 223, it is such a forgiving round, tolerates a bunch of clearance, and gives so few problems (usually).
> any 223 I ever owned always liked good pressure. I use Hodgdon data, pick a charge 1/2 way between starting and max. and call it a day, (generally). I drop charges from the measure, into the case. Lest you think I'm some sort of Bubba, I do however weight sort my cases, uniform the flash holes, uniform the primer pockets, the whole 9 yards. Weighing the powder charges never seemed to get me anything (ball powders).
> Sometimes switching to a Rem 7 1/2 will get you a bit more accuracy.
> I don't fool much with OAL, getting the bullet close to the lands, mostly because the magazine didn't allow it. So I quit caring about that.
If you are in the zone, the 223 is so easy to load for. Outside the zone it will drive you nutz. Starting loads were my nemesis, especially with ball powder. Also had a MMR (Mossberg AR) that shot 6" > 15" patterns until I figured the turd out. Now it shoots just over a MOA, 5 shot groups, in about 15 seconds, with CFE 223 and Hornady bulk 55gr with cannelure.