AR-15 going F/A

[02nightrain]

I have a PSA kit on a Anderson lower. Got the standard PSA FCG in it. After I built it back in the fall I took it out in the yard and ran 3-5 rounds through it. Worked fine. Then on New Years Eve I thought I would save some range $ and run a few rounds through it for a second test. GLAD I DID!. The thing ran 2-3 rounds full auto and jammed. I took it inside to the light. There was an unfired round chambered with a light strike on the primer. I cleared it an tried again and got a 5-6 round F/A string & another stop. Next day I broke it down. Everything looked ok. I had suspected a bent firing pin jamming and firing like open bolt ( as the gun was new). but that was not the case. I did the usual tests. With the trigger pulled the hammer would not lock. with the trigger released it would lock. BUT the difference in the location of the disconnector hook with the trigger pulled or released was maybe 1/16" I can not believe that it works at all. I broke open a couple more AR's for comparison & could find no difference except the depth of engagement with the disconnector. This IS a home build lower But I'm an ex Glock employee & FFL owner and have built my share of guns. I have not pulled the FCG yet thinking that I might just buy a Timminy or Giselle ( sp?). But I have not built that many AR lowers and might have just made a stupid mistake. Advise?

 

[wiscoaster]

I doubt you made a "stupid mistake" - it's obvious you know what you're looking at at what you're talking about. Time to take advantage of PSA's "lifetime" full warranty program, I think.

https://palmettostatearmory.com/help-center/warranty-policy.html#warranty-policy

 

[derek45]

I'd check the disconnector spring first.

you probably accidentally swapped it with the bolt catch spring during assembly

you can leave the disconnector spring in the bolt catch

 

[Cemetery21]

If the correct disconnector spring is in the trigger (a tapered coil spring with the fat side down), you can replace the fire control parts, try a different disconnector, or try to adjust the disconnector. To adjust, take a little metal from the front foot. That moves the hook of the disconnector closer to the hammer. What you are looking for (with the trigger NOT pulled) is for the hammer to kiss the disconnector on the way down, but kiss and pass it on the way up (not hook and stay) and catch on the sear.

 

[ApacheCoTodd]

Not an uncommon problem with quality, but worn - or - poor quality components.

Take the hammer/disconnector out, mark them in some way and replace with known performing units from one of your other guns.

If all is well, reinstall your old hammer with a replacement disconnector to confirm the hammer.

A legal point worthy of note is that if you have a rifle firing more than one round per squeeze of the trigger through componentry failure, you have until the next opportunity to remedy the failure to get it done. If you seek remedy and it continues to fail, you then have until your next - different solution - opportunity to remedy.

If you do not seek to remedy the situation or run out of viable optional solutions, you MUST disassemble immediately.

Before the pooh-poohs start, I have in fact been a Federal defense, subject matter expert on this exact scenario.

Todd.

 

[02nightrain]

Not an uncommon problem with quality, but worn - or - poor quality components.

Take the hammer/disconnector out, mark them in some way and replace with known performing units from one of your other guns.

If all is well, reinstall your old hammer with a replacement disconnector to confirm the hammer.

A legal point worthy of note is that if you have a rifle firing more than one round per squeeze of the trigger through componentry failure, you have until the next opportunity to remedy the failure to get it done. If you seek remedy and it continues to fail, you then have until your next - different solution - opportunity to remedy.

If you do not seek to remedy the situation or run out of viable optional solutions, you MUST disassemble immediately.

Before the pooh-poohs start, I have in fact been a Federal defense, subject matter expert on this exact scenario.

Todd.

Thank guys! I cannot emphasize how little movement I see on the disconnecter between trigger back/forward... It's almost imperceptible... maybe 1/100 inch. I cannot believe that it actually catches on the hammer. .. ANd not that light of a pull ! I'm guessing 5-6 lbs. usual Mil spec. Had the tree guys destroying my yard this week and I am in Georgia - so i've had the election/voting BS to deal with so I haven't been able to tear it down yet.

 

[H&R Glock]

I had a similar problem with a new PSA built lower on a new ARStoner 9mm upper. Over the course of a low number of rounds it started to go full auto on me. Anywhere from 2 to 4 rounds.
PSA was happy to take the lower back and rebuild the FCG. NO charge to me. Now it works as it should. It was fun while it lasted.

 

[grogetr]

I had a similar issue. I sent my trigger parts off to have them worked on to lighten the trigger pull. Mine would multi fire when I would slowly release the trigger to reset it. I messed with it a little but couldn't fix it so sent it back and had them redo it or maybe they gave me a different one. Everything worked fine after that. I'm not a gun smith by any means.

 

[edwardware]

. . . could find no difference except the depth of engagement with the disconnector.

TL DR: Recut the disconnector forward tab shelf so the disco engages further onto the hammer hook.

Long version:
It sounds to me like it's bumpfiring. I say this because I've induced the same failure by pushing the edges of trigger tuning; by reducing engagement you can set a trigger to be incredibly sensitive to should muscle tension and bumpfire if you don't hump up hard behind the stock.

If you set the disconnector to engage further (look at other ARs for an example) and make sure your disco spring is not weak, it should stop.

 

[jmorris]

This IS a home build lower

Home build as in you machined the receiver or you bought a lower receiver and put the parts kit in it?

 

[NIGHTLORD40K]

It sounds like the disco is the problem, but dont forget to try a different brand of ammo as well to rule out slam-fires. Could have got a batch of soft primers. It happens.

 

[Bcwitt]

I think I would try to pull the guts out of a known good rifle & try them in there first. That saves a lot of guess work.

 

[Sirclintler1]

What kind of ammo are you using? Which trigger assembly are you using? How does the BCG fit the upper?