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Installed a new free float M-Lok handguard the other day and went out to the range to check my zero this morning. Had a sandbag rest and was ready to check.... then.... BRRRRRRRRRRRP!!!!!!
12 rounds out of a full 15 round mag just like that, holy crap!

Never a problem with this carbine ever before after 700-800 rounds or so. No modifications to the fire control group, springs or anything. Totally stock PSA 9mm carbine that I put a MagPul UBR butt stock on and a different M-Lok fore end.

Any ideas on what to look for? I checked that the disconnector is working properly and everything seems to be in order with the fire control. No obstructions in the chamber, gun was clean before firing this morning. What am I missing here? Any help or suggestions would be appreciated!
 
One main issue with these AR based units is the amount of mass going forward as fast as it does. In other words, the speed of a heavy bolt traveling forward can cause the sear to disengage. Lots of folks have issues with light triggers and this as well. The second one I built was double firing. I tried different buffer weights, stiffer springs. I ended up with a captured buffer spring by JP Enterprise. That fixed it.
 
Also you said you swapped to the UBR? So you replaced the buffer tube. Perhaps the buffer is traveling further than it did, allowing the buffer to get a little more speed than it did originally.

Just a guess.

But I do know that the AR based pistol calibers are known for double tabs and going happy because the buffer weight going forward disenganging the sear.
 
I don't know, But when you check to see if your actions remedied the issue I would do it at a very private place in case it didn't. The last thing you want is someone thinking you modified it to fire full auto when you are in fact having a malfunction
 
I don't think it is the UBR because I have used it with that stock without any problems previously, but I'll check into that too! Just seemed weird that it started acting up immediately after the new float tube was installed. I was shooting some of the Winchester white box 124 grain nato ammo, is that stuff hotter? I usually shoot 115 grain ball through it...
 
I don't think it is the UBR because I have used it with that stock without any problems previously, but I'll check into that too! Just seemed weird that it started acting up immediately after the new float tube was installed. I was shooting some of the Winchester white box 124 grain nato ammo, is that stuff hotter? I usually shoot 115 grain ball through it...
That's definitely a hard one to figure out.

I know they have issues with bolt mass and sears disconnecting, that's why I mentioned it.

I ended up selling all my AR based 9mm's as I just couldn't get them right.

Keep us updated.
 
I don't know, But when you check to see if your actions remedied the issue I would do it at a very private place in case it didn't. The last thing you want is someone thinking you modified it to fire full auto when you are in fact having a malfunction

Load one round with a snap cap behind it, you'll know if it went wrong if the hammer is down on the snap cap.
 
Also you said you swapped to the UBR? So you replaced the buffer tube. Perhaps the buffer is traveling further than it did, allowing the buffer to get a little more speed than it did originally.

Just a guess.

But I do know that the AR based pistol calibers are known for double tabs and going happy because the buffer weight going forward disenganging the sear.

Measured the buffer tube internal length on the UBR stock vs. the original CAR buffer tube that was on the gun. Identical lengths with the buffer spacer installed in the UBR, so I don't see how the buffer could be traveling any further and gaining any more momentum to cause the problem. Maybe the UBR tube is larger diameter causing less drag on the buffer/spring?

Tried to make disconnector fail by letting bolt fly forward while keeping trigger depressed (on an empty chamber) and could not make it fail. I tried locking the bolt back and hitting the bolt release as well as pulling the charging handle back and letting it go the full stroke. I know it is different under recoil, but I thought I would see if I could make it fail without having to actually fire the gun.

Don't know what changed since I only changed the handguard since the last time I used it without any problems.
 
Thanks for the ideas guys.
Don't they have to prove intent for the prison time? l certainly did not want this to malfunction like it did. Good thing I was wearing safety glasses! I still caught a piece of brass debris in the cheek when it fired (assumingly) out of battery on the last shot rupturing the case. Front half of the case and the bullet were still in the chamber. Case head and some fragments laying on the bench. Just trying to get some help when everyone is closed for the holliday weekend.
 
Thanks for the ideas guys.
Don't they have to prove intent for the prison time? l certainly did not want this to malfunction like it did. Good thing I was wearing safety glasses! I still caught a piece of brass debris in the cheek when it fired (assumingly) out of battery on the last shot rupturing the case. Front half of the case and the bullet were still in the chamber. Case head and some fragments laying on the bench. Just trying to get some help when everyone is closed for the holliday weekend.

Dunno, but in the meantime would you want to have to mortgage your house to defend such?...

..
 
There was a guy prosecuted for a malfunctioning "full auto" AR some years ago. I think there were more things going on in that case however in any case a full auto firearm is one that fires more than once with a single pull of the trigger. That's what you have, so....

Just be careful.
 
Fiddling with it some more I think I have it figured out.

While holding the lower in my hand (upper removed) I can cock the hammer, and pull the trigger which releases the hammer like it should. While keeping the trigger pulled I can cock the hammer back and it holds under tension of the disconnector like it should. I can push down on the hammer and let go quickly while holding the trigger and the disconnector holds like it should. BUT when you drag your thumb across the hammer towards the butt stock the hammer releases, consistently every time.

This gun has one of the "enhanced" trigger groups from Palmetto in it as an OEM part (it came this way). I think the hammer trigger disconnector is bad. It shouldn't let go like that. I will give Palmetto a call on Monday and see what they will do to fix this.

Glad I figured it out, I think this was going to drive me crazy if I didn't.
 
Fiddling with it some more I think I have it figured out.

While holding the lower in my hand (upper removed) I can cock the hammer, and pull the trigger which releases the hammer like it should. While keeping the trigger pulled I can cock the hammer back and it holds under tension of the disconnector like it should. I can push down on the hammer and let go quickly while holding the trigger and the disconnector holds like it should. BUT when you drag your thumb across the hammer towards the butt stock the hammer releases, consistently every time.

This gun has one of the "enhanced" trigger groups from Palmetto in it as an OEM part (it came this way). I think the hammer trigger disconnector is bad. It shouldn't let go like that. I will give Palmetto a call on Monday and see what they will do to fix this.

Glad I figured it out, I think this was going to drive me crazy if I didn't.
Swap the trigger. Send that one back to PSA.
 
Trigger is out, gonna see if Palmetto will replace it. I would think they would when I tell them what their "enhanced" trigger did....

Thanks Fellers!
 
I don't think it is the UBR because I have used it with that stock without any problems previously, but I'll check into that too! Just seemed weird that it started acting up immediately after the new float tube was installed. I was shooting some of the Winchester white box 124 grain nato ammo, is that stuff hotter? I usually shoot 115 grain ball through it...
Winchester ammo has soft primers IICR. Free floating firing pins and soft primers can lead to this type of malfunction. Do 9mm ARs use a spring loaded firing pin or still use a floating pin?
 
Winchester ammo has soft primers IICR. Free floating firing pins and soft primers can lead to this type of malfunction. Do 9mm ARs use a spring loaded firing pin or still use a floating pin?

Mine has a spring loaded firing pin. Not sure on other 9mm AR's.
 
I had a Walther PPK/S do that to me once. Interarms was more than happy to fix the problem free of charge since I enclosed a pic of the slide laceration on my hand.
 
Just send it back to PSA and write them a letter about the problem. (Keep it between you and PSA).

Do NOT let the Feds get wind of this. They may attempt to nail you for "constructive possession". If you ship it back to PSA, it will "preempt" any possibility of illicit possession. The lack of intent will be evident when you return it to the manufacturer with a letter of explanation/complaint.

No "mens rea" (guilty mind), no intent. Since there's no intent, no criminal case.
 
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