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A week or so ago we were teaching an M240b class to a group of EOD guys out at a munitions range in the panhandle of Florida. We shot about 1300 rounds per student and had 30 students. We fired from the Bipod in the prone position, from a tripod with the T&E attached and then mounted to a Humvee. There was TONS of brass laying around and because of the amount of rounds being fired, most of it was very hot.

One of the students at the Humvee station had a stoppage, which her cleared correctly by pulling the cocking handle to the rear. The ejection port on the M240B is on the bottom of the receiver and the live round landed with the rest of the brass. Well, with his jam cleared he put the cocking handle back forward and continued to fire. Only a few moments after he started firing again the round in the pile of brass cooked off from the heat coming off of the fired casings scaring the holy bejeebus out of the instructor and student.

The case exploded, sending the primer out the rear of the casing, fragments everywhere and even launching the bullet with enough force to take a little chunk out of the turret the bullet even mushroomed out a bit from the impact. Fortunately no one was hurt, and we were able to laugh it off, but it certainly could of turned out a lot differently.

I've seen a round cook off inside the hot chamber of a few different machine guns but never just from the heat of the fired casings. So just in case you ever wondered what it takes for a round to cook off, now you know.

Oh and just to brag, we topped off the day by firing a few hundred rounds out of the M107 (Barret .50 cal semi-auto rifle) at tanks and old fuel trucks...which is probably one of the funnest things I have ever done.

are you SURE the round "cooked off", and it was not due to a "slow burn".

I'm not doubting your assessment, but I would look at other possibilities before starting the "cooking off" theory too much.

Well, its a 7.62 Nato round and it was close to a minute between the stoppage and the round exploding. I'll leave it to someone more edumacated than me to decide, but that seems like an awful long time for a slow burn.

could be an awful long time at a full minute. I had always heard 30 seconds was considered safe to eject. Interesting. I still have a hard time with the whole cooking off thing, but you guys do lay down a pretty good pile of hot brass. Didn't Box O' Truth have a thing about cooking off rounds some time back?

I too am interested in the answer.


surv

mr surveyor wrote:could be an awful long time at a full minute. I had always heard 30 seconds was considered safe to eject. Interesting. I still have a hard time with the whole cooking off thing, but you guys do lay down a pretty good pile of hot brass. Didn't Box O' Truth have a thing about cooking off rounds some time back?

I too am interested in the answer.


surv

Yeah, but as I recall the cooked off rounds were as a result of shooting holes in AK mags full of ammo. Maybe there's one I missed, but that's the only one I recall that had rounds cook off.

http://www.fireangel.biz/v-Mythbusters+ ... BX1kvJVrjc" onclick="window.open(this.href);return false;

I checked Box O' Truth...couldn't find what I was looking for. Also googled and found a lot of hits..one particularly from an old post on The High Road describing tests noted in a book where it seems that 275 deg F was about the low end temp described for a "cook off" to occur. I still wonder if it could have been a (really) slow burn?

How long was the round in the chamber? Thats probably where the heat came from if it did in fact cook off, and it wasnt due to a slow burn. I have to doubt that laying next to hot brass on the ground could have caused it. Regardless of the cause, Its good to hear that yall were able to laugh it off and noone got hurt.

This old Navy gunner's maxim is that hangfires happen when they want to.