Was the upper cerikote or similar or blued?
9mm case blown apart,Why?
- Thread starter RFD
- Start date
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Was the upper cerikote or similar or blued?
GuitarmanNick
Ultimate Member
On a pistol caliber AR, there is no lock up to hold the bolt closed when firing like in the rifle calibers. It is a simple blow back design. If the gun is fired too fast, it can indeed lead to an OOB when the bolt bounces toward the rear and there is not enough pressure to hold it closed. My pistol had a 5.3 oz. solid buffer and heavy .308 spring but that is apparently insufficient when speeding up the firing rate. I thought that the build was okay for what I was doing, but was obviously mistaken. Fine for slow fire, but a buffer around 8 oz. would be better for a 10.5" pistol.
For faster rate of fire, I would need to add a spacer to take up the over travel in the bolt, and install a heavier buffer with a sliding weight.
From the reading I have done since this happened, OOB detonation in pistol caliber ARs is fairly common when rate of fire increases, unless steps similar to those described above are taken. They can be made to function fully automatic and be reliable, but may take some tuning.
It may soon be offered in the classifieds but still contemplating my options. Sure was fun while it was running, but not sure I want to risk having another OOB problem. If a semi-auto cannot be fired rapidly without issues, I do not consider it to be safe.
For faster rate of fire, I would need to add a spacer to take up the over travel in the bolt, and install a heavier buffer with a sliding weight.
From the reading I have done since this happened, OOB detonation in pistol caliber ARs is fairly common when rate of fire increases, unless steps similar to those described above are taken. They can be made to function fully automatic and be reliable, but may take some tuning.
It may soon be offered in the classifieds but still contemplating my options. Sure was fun while it was running, but not sure I want to risk having another OOB problem. If a semi-auto cannot be fired rapidly without issues, I do not consider it to be safe.
GuitarmanNick
Ultimate Member
Cerikote like most.
IX-3
Active Member
For what it’s worth the Foxtrot Mike Heavy 9MM buffer (6.4oz) has worked great in my 9MM so far. It’s longer than a standard buffer so you don’t need a spacer. I only have a few hundred rounds through it so far but I’ve done most of my shooting at a faster rate of fire.
Last edited:
GuitarmanNick
Ultimate Member
What barrel length?
IX-3
Active Member
10.5"
The safety problem with most semiauto 9mm ARs is that they used a ramped bolt like Colt did in their machine guns. Yes, a ramped bolt slows the hammer reset speed, limiting trigger slap but without an auto sear to prevent from dropping the hammer too early it's dengerous. Colt didn't have that problem. A non-ramped bolt prevents the hammer from reaching the firing pin if you are out of battery a little.
Bolt bounce isn't likely causing the problem. A bolt doesn't bounces a ton if you watch it on high speed video. Not enough to pull a case halfway out of the chamber. It's *FAR* more likely that you can either pull the trigger too fast and drop the hammer before you're in battery OR there is a small piece of debris or a tiny dent in the round preventing it from seating fully. The latter can happen w/ any amount of bolt weight even shooting very slowly. So a heavy buffer is not a cure all.
The only real way to help prevent semiauto AR9s from firing out of battery is to use a non ramped bolt. Then use a heavy buffer to limit the trigger slap.
Bolt bounce isn't likely causing the problem. A bolt doesn't bounces a ton if you watch it on high speed video. Not enough to pull a case halfway out of the chamber. It's *FAR* more likely that you can either pull the trigger too fast and drop the hammer before you're in battery OR there is a small piece of debris or a tiny dent in the round preventing it from seating fully. The latter can happen w/ any amount of bolt weight even shooting very slowly. So a heavy buffer is not a cure all.
The only real way to help prevent semiauto AR9s from firing out of battery is to use a non ramped bolt. Then use a heavy buffer to limit the trigger slap.
ralph.mclean
GOC (Grumpy Old Cop)
Either out of battery, or overloaded with powder which had not completely burned by the time the ejection cycle started...
It appears to be an out of battery, or over pressure load. Just for FYI I stay away from shall we say bargain ammo. It cost more but I've never had this happen in any of my firearms. Hope this helps.
ironhead7544
Active Member
- Oct 27, 2018
- 188
smdub is correct about the original Colt 9mm AR-15. It also had a different hammer.
You need the 9mm buffer. If you have a standard length 9mm buffer, you can put $1.75 worth of quarters in the buffer tube for a spacer. You should have just enough spacer to allow the bolt catch to work. This is with the Colt type magazine setup.
There is a lot of info on the AR-9mm out there.
You need the 9mm buffer. If you have a standard length 9mm buffer, you can put $1.75 worth of quarters in the buffer tube for a spacer. You should have just enough spacer to allow the bolt catch to work. This is with the Colt type magazine setup.
There is a lot of info on the AR-9mm out there.
Not saying this is or is not an OOB, but you can have the situation where a catastrophic case failure looks a lot like what one would consider an OOB. It's the situation where a case head fails all the way around separating from the body. The body of the case that's in the chamber then get's pried out of the chamber and flares out looking for all the world like an OOB.
Doco Overboard
Ultimate Member
Maybe that particular cases had an overly sensitive primer and lit off with a light firing pin strike or a pin that was stuck in the port due to being partially stuck. A carbon infused chamber will do that on some rifles that have a bridge because because of FP dwell timing alone. As soon as the cartridge met mechanical chamber resistance the fp protrusion may have lit it off a little prematurely.
Thats why its a good idea to wear safety glasses when shooting no matter what.
Thats why its a good idea to wear safety glasses when shooting no matter what.
I had a similar issue with my 9mm AR. The barrel chamber bevel was a little rough from machining and caused the case to stick and the bolt to not go into battery fully. When I pulled the trigger there was a pop and a lot of gas and debris came out of the ejection port. When I pulled the case out there was a 2mm wide slit where the chamber/bolt was apparently not containing the pressure, and the gasses and powder come out from there. The bullet was seated about halfway down the barrel and was easily tapped out with a cleaning rod. I'm not sure why the hammer was able to set off the primer while not in battery but it happened. I removed the barrel, polished the feed ramp/bevel, and it's been flawless since. I'm guessing there was just enough of the free floating pin exposed for the hammer to make contact and detonate the primer. I'll upload a pic if I can find it.
What you are describing is case head separation and usually occurs when there is too much headspace. You see this more on rifle caliber cartridges rather than pistol caliber.
GuitarmanNick
Ultimate Member
I have not yet seen a 9mm bolt for an AR with a free floating firing pin. The bolt moves much faster in a blow back system than in a normal AR gas system. A spring is the only thing keeping most from firing fully automatic as long as the trigger is held back.
Oh, it can happen on pistols too. On most pistols it requires faulty brass, otherwise the feed-ramp will blow out first. Better supported pistols...
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