My first AD just happened

[NavyVet1959]

I had reloaded some rounds for my .300 AAC AR and wanted to check the cycling. First one I tried, the bolt would not go all the way forward, but I could not see any marks on the bullet where it might be touching the lands of the rifling, so I dropped it in the chamber to see if it would seat all the way. It looked good as far as I could tell, so I dropped the bolt on it and then it fired. I was sitting on the couch at the time and barrel was resting on the floor with my feet beside it. Blasted a nice hole in the carpet and a bit of concrete broken out. Concrete/lead splatter hit me in the feet, ankles, and inner arms, but mostly like being blasted with sand. Wasn't wearing my glasses, but nothing major hot in the eyes. All in all, I'm pretty damn lucky.

[sparky45]

Glad you're OK, that must have gotten your blood pressure cooking in no time. I to have had some trouble getting 300 BO reloaded ammo to chamber and I've come to realize the brass used to make the 300 BO is critical. Fella on here that sells the brass (Grumpa) can get you the proper info on what brass to use and it all has to do with the thickness at the neck. LC is my favorite to use as well as a few others, but there's lots of manufacturers brass that shouldn't be used, that is if your usage is in a AR platform. Bolt gun should be OK.

[rockrat]

Glad you and anyone around you are OK

[osteodoc08]

Hmmmm. a slam fire? Primers proud? If it wouldn't chamber, did it fire out of battery as well? Glad your safe and carpet/concrete can be replaced rather easily.

Perhaps our knowledgeable folks here can recreate what happened and have a better diagnosis of how it occurred.

[NavyVet1959]

The rest of the mag look good. Safety was on.

[RED333]

Primer brand and type?

[NavyVet1959]

Primer brand and type?

Not sure... I think it was Federal Small Pistol Magnum... At least that is what is on my reloading bench right now. I also use Tula and Wolf sometimes.

[Geezer in NH]

Federal pistol primers are one of the softest going. AR firing pin dents lots of rifle primers I can imagine it firing the fed small pistol especially with the bolt going forward full force not having to strip the cartridge from the magazine.

[gunauthor]

Glad you're OK. I damn near blew my foot off with a friend's 41 magnum. Did get my heart goin'.........

[quickdraw66]

What does the primer look like? Is there a primer strike, or is it flattened? IIRC, the safety on an AR only blocks the trigger, not the free floating firing pin. It's possible that the force of the bolt slamming home was enough to drive the firing pin into the primer and set if off. I've heard of slam fires in ARs before, but they tend to be rare. Soft primers can greatly increase that risk. It's also possible that gunk may have built up around the firing pin, holding it proud of the bolt face. Just some thoughts.

I'm glad you made it though that without an extra hole in you.

[NavyVet1959]

The primer had a raised center portion instead of a dent from the firing pin. That says to me that it wasn't hit by the firing pin and must have gone off when just the body of the bolt hit it.

Tried to duplicate it with my bullet trap as the backstop instead of my floor and it would not occur.

[oneokie]

A multitude of errors; Soft primers, free floating firing pin, closing the bolt on a chambered round.

[country gent]

The raised portion of the primers center is where the firing pin would have been if forawrd and had fired it. More than likely a slightly proud primer and hit of the bolt face was enough with the softer pistol primers cup. Also check bolt face for a burr or raised edge around firing pin hole, this can also aid in slam fires. Another thing to check is the lower corners of the bolts locking lugs for chips or rounding. The same area in the barrel extension should be checked but is much harder to see the faces of the locking lugs there. Am happy to hear no one was seriously injured, following safe practices is always good. Another area to check before firing is the muzzle for obstructions. Having the rifle resting on the floor solidly also may have contributed as it didnt allow the force to flow forward as it would with the rifle unblocked.

[NavyVet1959]

A multitude of errors; Soft primers, free floating firing pin, closing the bolt on a chambered round.

Well, I was needing to figure out why the bolt was not going all the way forward. Dropping it in the chamber looked like it was going all the way in,

Turned out that I need to change my OAL from 2.255" to 2.058" for it to chamber all the way with this bullet profile.

[oneokie]

Tried to duplicate it with my bullet trap as the backstop instead of my floor and it would not occur.

Most likely reason it did not reoccur is the orientation of the rifle. You imply in the op that the muzzle was on the floor with the rifle in a vertical orientation. Trying to repeat the AD with the rifle in a horizontal orientation changes inertia forces.

[petroid]

I have unchambered live rounds and noticed dents in the primer with even CCI SR primers. I have switched to CCI 41 primers for all my AR ammo.
It very well could have been a "proud" primer, but either way, I think a bit harder primer cup can reduce the chance of an accidental discharge.
Glad you are not injured.

[RED333]

Most likely reason it did not reoccur is the orientation of the rifle. You imply in the op that the muzzle was on the floor with the rifle in a vertical orientation. Trying to repeat the AD with the rifle in a horizontal orientation changes inertia forces.

Good point here, with the rifle muzzle resting on the floor, the snap of the bolt going home,
no where for the rifle to move "out of the way". The floor stopped the rifle from moving.

[NavyVet1959]

Most likely reason it did not reoccur is the orientation of the rifle. You imply in the op that the muzzle was on the floor with the rifle in a vertical orientation. Trying to repeat the AD with the rifle in a horizontal orientation changes inertia forces.

Perhaps, but I'm not all that inclined to try to shoot my foot off again tonight.

[shooterg]

I useta chamber all rounds to be used in rapid fire stages of HP in the AR it was gonna be used in before a match - BUT I took the firing pin out first. Glad all is OK and hope it's also your LAST AD !

[cuzinbruce]

"Turned out that I need to change my OAL from 2.255" to 2.058" for it to chamber all the way with this bullet profile."

That seems like a rather large change in the OAL. Almost 2 tenths of an inch.