Round "cooking off" in the magazine?

AR/M4 AR Technical Discussion
1

A few weekends ago I was at the local range and some guys were in the other bay shooting AR’s. One of them came up to the line and started firing.

As he squeezed off a round, his magazine floorplate was blown out and the remaining rounds, follower and spring was dumped out. the RO immediately commanded him to stop and they went through the clearing process etc.

It was though that a round cooked off in the magazine; others thought something sharp probably hit the primer, setting off the round.

No injury to anyone and the magazine was re-assembled without any apparent damage.

What do you think happened?

2

Kabooms will follow the path of least resistance which is down the magwell and out the bottom of the magazine. Sounds like a normal kaboom to me, nothing special.

Also setting off a round outside the chamber is pretty unimpressive, its just a big firecracker.

3

Sounds more like the weapon fired out of battery.

4

Cb and Troupe are on point.

5

It appears to me that he didn’t reassemble his magazine properly and firing the weapon launched the floor plate. If a round went off while in the magazine there would be damage, at least to the magazine, possibly the lower as well. Of course a round being ignited without a chamber doesn’t generate any real velocity. Or it did fire out of battery? In the two cases over the years of seeing this there was evidence of it happening.

6

Thanks for the inputs, guys.

If it fired out of battery, would the bolt carrier still be able to extract and eject the shell casing? Unfortunately, I didn’t see if it did.

I saw the magazine; I didn’t see any damage to it so it.

7

How does an AR fire out of battery?

8

It didn’t.

The AR can not fire out of battery.

9

If the bolt is out of spec or the headspace is improperly set, an OOB detonation is completely possible resulting in a case head separation, blowing out the magazine and (sometimes) destroying the upper reciever. I have seen it happen.

10

Something similar happened to me with Canadian milsurp rounds. Round fired in battery. Way over pressure. Blew the bottom of the bolt carrier down through the magazine with all the rounds, spring, follower, floorplate, etc. The top of the bolt carrier had a longitudinal crack so that the key for the gas tube and the bolts just fell out when I finally got the upper apart. The upper receiver bowed out at the ejection port and bent the dust cover and rod.

Needless to say but that pretty much ended the day.

11

Thanks for squashing that. I hate that myth.

Sounds like a small kaboom… that could easily appear to be a mag cook off to an RO. They don’t get that job for being the sharpest tool in the shed.

12

Did the shooter and RO find the round fired just before the alleged cook off? Probably just a typical blown case.

As for an OOB firing, isn’t it impossible for the firing pin to protrude far enough to reach the primer until the bolt is locked?

13

The firing pin doesn’t protrude enough from the bolt face until the bolt locks into battery to ignite a primer.

14

No. Not with the AR/M16 FoW. Not possible.

What you sawr was a ruptured case head. Happens a lot with worn brass and crappy reloads.

15

Thanks for the con-ed, guys. I’ve heard of OOB’s in this weapon type but never saw one in person. Makes sense that OOB would be highly unlikely in this platform.

16

Also the hammer can’t make contact with the head of the firing pin when it’s out of battery.

17

My comment was a bait cast… thanks for making my point without me having to be a dick…

18

serious amount of lead down range in short order to get to cook off territory. I wouldn’t think that a cook off from an M-16 type weapon would be all that bad because

  1. It’s a closed bolt weapon and
  2. It’s not belt fed. (far less chance of run away if you have a malfunction you can’t clear) Just take the mag out keep the weapon pointed in a safe direction. For us, we wait 3-5 seconds but only have 10 to clear it.
19

Not a cook off in the chamber…but…in the mag. Or set off by a sharp pointy thing in the mag. It’s pretty unanimous that it was a blown case.

20

So I was able to track down the fellow who had this incident. He said that the round in the chamber didn’t cook off; one of the rounds in the mag went off. He said that shell casing has a gash about 1/8 inch long perpendicular to the extraction groove. He said it was a reloaded round.

It looks like they’ve reloaded that shell one too many times.