I wasnt firing an AR, but a CZ 527 bolt action rifle.  Today, I wanted to test some Wolf .223 Rem, 55-gr HP ammo I picked up.  First round out of the box, I try to chamber it and it jams about 1/4" before it fully chambers.  I can't extract it, so i tap it out with a cleaning rod.  I noticed marks around the bullet about 1/4" down from the bullet tip, but didn't realize what they were at the moment.  So, I set that round aside as it was suspect and grabbed another one.  I chambered it with a little extra force, but not an unreasonable amount, and BOOM!  The rifle fired as I closed the action!  That ended my shooting for a few hrs, and my testing of the Wolf ammo.  After a few hrs, I got my courage back up and went back to shooting, only this time using my reloads.  They chambered and fired perfectly.

So, thinking back, I've realized that those marks were the rifling engraving on the bullet.  By jamming the bullet into the rifling it somehow caused the rifle to fire.  But, the remaining rounds all mike under max OAL of 2.260" (~2.250"), so they aren't loaded too long as far as I can tell.  However, my reloads are at 2.188" and they chamber perfectly, never any rifling marks on unfired rounds I extract.  I'm stumped as to why this round fired like it did, but suffice to say, I'm not shooting any more Wolf ammo anytime soon.

Was the primer struck?  That means the striker was released, which happens when the trigger is pulled.  Or the striker is broken and the firing pin sticks out the face of the bolt.

-- Chuck

I saved the fired case, and yes, the primer was struck; nice central indentation on the primer.  However, I fired 20 rds of reloads about 2 hrs after the incident with the Wolf ammo and they worked perfectly with no premature firings.  Therefore, I dont think there is a problem with the firing pin being broken or damaged.

Umm - does your CZ have a set trigger?

link to manual........
            [url]http://www.cz-usa.com/_d/all/Instruction_Manual_CZ_527.pdf[/url]

Yes, it has the set-trigger feature, but it was not engaged at the time I chambered the round.  When not set, the trigger breaks at ~3.5 lbs, not unreasonably light by any means.

"slam-fire" can also be caused by a high or improperly seated primer.

A heavy firing pin striking the primer can cause it to slam fire from normal loading, or if the firing pin gets stuck in the fire possition it could fire from loading it