9mm AR went full auto today

[blaster113]

I swapped out the stock CMMG trigger group with an ALG Quality Mil Spec trigger, and been having intermittent doubling. I also put in a "redneck" buffer tube spacer consisting of 1.75 in quarters. Buffer is a CMMG 9mm. Thought it was the quarters causing the doubling, so I took the "redneck" spacer out. It worked ok for about 75 rounds, then today it went full auto on me twice, both times emptying out the mag. Good thing I only had 5 rounds in each mag. Yanked out the ALG trigger and the stock unit went back in.

Anyone have this happen? Did I get a lemon trigger unit? thought ALG made good shit.

The ALG trigger group is a mil spec unit that is polished a little. Springs felt a tiny bit softer than stock. Everything was installed per spec, including the springs. I purposely chose this unit as I've read that full auto can happen with triggers that are too light in blowback ARs. Pull weight is advertised at about 6.5 lbs.

Bottom line; I'd think twice about screwing with the FCG of a 9mm AR. If you got a stock trigger and its decent, leave it. Lesson learned.

(And if starting a new build buy one of those extended, heavy 9mm AR buffers. I'm ordering one this week.)

Edited July 28, 2016 by blaster113

[Southpaw]

Did you possibly install the disconnector spring upside down? The larger end should face down.

[Hi-Power Jack]

Sounds like an inexpensive way to get a "full auto" weapon - I'll buy it from you

[StealthyBlagga]

Switch out the light springs for full-power GI hammer and trigger springs - I have found both are necessary for maximum reliability on a 9mm AR. If you want a decent trigger at a reasonable price, do what I did and install a JP trigger under your existing GI hammer (with the above mentioned GI springs)... pull is heavy but short and crisp, and IMHO about the best you can do while maintaining robust reliability. I also run KNS pins because the regular GI pins are prone to breakage with blowback guns.

Sounds like an inexpensive way to get a "full auto" weapon - I'll buy it from you

10 years

Edited July 28, 2016 by StealthyBlagga

[Dannybot]

I have been considering a 9mm build. Some bolt manufacturers say use milspec FCG's only to avoid going full auto....

[blaster113]

The disconnect and all other springs were installed per spec. My guess is the tolerances on the ALG were a little loose thus I had the doubles initially. After the parts started to break in the tolerances opened up, and voila 5 round burst. Worst part is the first burst happened when my 8 year old was on it. Scared the shit out of both of us.

All this aggravation for a trigger unit that was only marginally better than the stock CMMG, which for a basic LPK had a decent trigger. The ALG unit is supposed to be mil-spec that is cleaned up. Very disappointing. I'll try the JP trigger spring and KNS pins with the CMMG parts and see what happens. The KNS pins are supposed to help the trigger pull too. My kids won't touch the gun again till I'm 100% sure the damn thing is safe.

This may be blasphemy but my attitude toward full auto is "meh". I've shot enough full auto at work that I want no part of it out of my semi's hehe.

[steviesterno]

full auto is fun but only when you're expecting it :-/

[ryan45kim]

I have several ALG triggers that are working great, I would guess you got a Lemon, I'd call ALG and see if they will make this right.

I've also been running a full JP trigger group in my 9mm ar (although I do use the red hammer spring). I have a little over 2k rounds through it so far, only malfunctions I had were when I tried to use the mag as a monopod.

[blaster113]

Were any of the ALG triggers in a 9mm, or only the JP unit?

[NickBlasta]

Make sure you clean the firing pin channel on the bolt real well, too. It tends to fill with fouling quite quickly and the firing pin can get stuck forwards and slamfire.

Edited July 29, 2016 by NickBlasta

[blaster113]

I cleaned the firing pin channel a short while ago. Man blowback AR's sure get dirty quick.

[dynamicalflinch]

It could be a few things causing the problem, but quarters are not an acceptable spacer. Look at photos of a correct spacer, it not only has space in the rear, it also has an insert that sits inside the spring and gives the buffer a space to contact without bottoming out the spring. With or without the quarter, it's not correct and will cause excessive wear and poor reliability. Spacers are CHEAP (almost as cheap as people who choose not to use them). Blows my mind doing why so many think it's kosher to run a gun MISSING a part of the recoil system.

I'm sure some will say "it works...", just like a car will work with flat tires and low/no oil.

[MikeRush]

I am running the ALG ACT with the provided springs in my blowback and liking it quite a bit.

[ryan45kim]

No my ALG triggers are in 223 guns. I'm running JP hammers and triggers in my 9mm ar and my match gun in 223. I do use the red hammer spring for all my JP triggers.

[Patrick Scott]

I ran an ALG ACT with JP reduced power springs in my 9MM AR for near 2k rounds. Never had an issue. For refference-JP bolt, JP308 buffer spring, Couple of different buffers( Colt and Hahn).

[blaster113]

Spikes ST-9x heavy extended buffer ordered this morning. Is a heavier buffer spring necessary? Have a regular carbine spring. Going to run KNS pins with the stock trigger for now.

Edited August 2, 2016 by blaster113

[bwikel]

I am running the Spikes ST-9x buffer with a wolf extra power carbine buffer spring, it seems to take some of the thump out of the recoil. It will run with a reg carbine spring as well.

[blaster113]

Update if anyone cares

Using the stock CMMG trigger parts I put in a JP service rifle 4.0 spring kit. Works as advertised. Takeup feels better, reset distance seems to have been reduced a little. No problems with ignition. I then put in a KNS anti-walk pin kit. Trigger pull improved even more. Pull weight is somewhere in the neighborhood of 5 lbs (my gauge only goes to 4.5). 100 rounds through the gun and yea so far no full auto . The CMMG group wasn't bad to start with. YMMV if you're starting with a shitty trigger. Hopefully this setup holds cause it feels nice.

also installed a Spike's ST-9X heavy extended buffer. With the new buffer when I pull the charging handle back the bolt barely clears the bolt catch at full extension. Perfect. Ejection used to be straight out the ejection port about 20+ feet; now the ejection pattern is slightly upwards, still straight out the port and distance reduced to about 10-15 feet. Dot movement improved. The dot now kind of lazily lifts a little then falls straight back down. There is a little more push to the rear during recoil now I'm guessing due to that 8 oz buffer moving to the rear.

Bottom line; if starting a new build I would definitely use an extended buffer from the start. No need to screw with spacers and dot movement is reduced.

in the pics the ST-9X is compared to the CMMG 9mm buffer

[dauntedfuture]

I had a 9mm AR at one point. When I initially built it, there was an issue with hammer follow. I don't recall whose 9mm BCG I had, but it did have a shrouded firing pin and a firing pin spring. I swapped out the disconnector and it worked without an issue. I suspect your trigger was not resetting fully. I see you got it working; good info. I gave up on the 9mm AR as it was not accurate, probably not a great barrel, and I was breaking trigger pins.

[2MoreChains]

The Spikes ST-9X buffer is what I am using along with a standard carbine buffer spring. I also went with the KNS anti-rotate pin kit, but for trigger I dropped in a Hiperfire that I had laying around. So far so good with ~2,000 rnds thru the gun.

Good luck!

[bdugger]

Just built up a 9mm PCC.  Was having issues with doubling (one trigger pull held to the rear and two rounds shot) 

I traced my issue to my buffer. I had 7ozs in the buffer with room for the weights to move (try to reduce bolt bounce/recoil) I wasn't only getting doubling but the consistency of bolt cycle was erratic. after I added more rubber disks so that the weights don't move, It's shooting consistent and no more doubling. Although I need to remove weight from buffer, it'll short stroke if not shouldered tightly.