I just finished my first 9mm build and am having an issue every few rounds (3-4) with the trigger not resetting.

The round chambers but the sear/dis-connector never appears to catch the hammer; this occurs when suppressed or unsuppressed. Changed mags, no difference.

I am shooting Winchester whitebox ball. May need to use a lighter spring and or buffer? Any suggestions?

Build config:

Forward Assist Gas Vent.
Tros 5" barrel.
QC-10 9mm bolt.
QC10 Small Lower
RRA mil-spec trigger group.
Spikes Tactical ST-9X 9mm Stainless Steel Buffer
Wolf heavy duty buffer spring.

Absolutely try a regular 9mm buffer or swap out the Wolff spring for a regular carbine spring.

I agree...I would  swap the buffer spring 1st. with a plain carbine spring....and maybe use a slightly lighter buffer.


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Thanks!....new buffer on the way...swapped out buffer spring.

If I shoot it a bunch the crappy mil spec trigger is gone too.

ALG Combat trigger.....

I see a couple of things to look at.  I'm completely behind getting an ALG ACT.  I have two of those wonderful triggers (both of them in my SBRs, one of which is a QC10 GSF lower).  But there are some key points in your post that indicate where to start.

Start by lubing the snot out of the gun.  The WHOLE gun, but especially the FCG.  And start the lube process by cleaning the living shite out of the thing.  Degrease it until the metal is bone dry, then apply copious amounts of your lube of choice.  You MUST clean the parts because guns and gun parts are NOT shipped "lubed;" they're shipped with anticorrosion coatings.  They may seem slick, but those coatings do NOT properly lube a firearm, particularly an AR-type firearm.

Dial back the heavy buffer and spring.  You want to resist the bolt's opening, not hinder it.  I have a 10" 9mm upper that runs like a top with a standard carbine spring and buffer.  With hot loads that's not particularly "fun," but it doesn't take a lot of extra mass to make hot loads smooth right out.

Get some grown up ammunition.  WWB 115gr 9mm is "mild" to say the least.  For not much more you can get much stouter stuff.   Just as with centerfire rifles, breaking in new 9mm ARs requires some full power rounds until all the rough spots are smoothed out.  That can take as much as 200 rounds with some guns.

Thanks for the input.

Initially the first round chambers and goes off; second round chambers but hammer is not reset.  Manually cycle, ejects 2nd round then will run for 3-4 rounds (both rapid and slow) then the hammer does not reset again. Lots of lube. To clarify this observation was the first fire test.  I have not retested since swapping the spring.

I've swapped springs and have a new buffer en-route should that not work.

Hoping its just spring/buffer. I will be able to test it fully this weekend. Have 300 rounds of WWB and 100 of the full power stuff.

Thanks.

What mags are you running?  Why I ask is because on my GLF the KCI 26rd mags are really tight side to side.  If it slam it in the mag goes too deep and interferes with the bolt.  So what I have to do is insert the mag then pull down.  Otherwise, the KCI mag works fine.  Will need to sand down the sides of the mag to the point they drop free.

Round two.

Replaced the buffer and spring with several combinations. Jams/failure to eject were frequent.  In the end it liked the standard recoil spring and Spikes Tactical ST-9X 9mm Stainless Steel Buffer the best.  Put about 1/2 a bottle of Fireclean (yeah I don't buy it anymore) in the action and the jams/lack of trigger resets went from every two rounds to 20 or so rounds while suppressed.

Switched to Glock factory mags. Lubed the chamber. Checked the chamber with q-tip no apparent burrs.

Put about 200 rounds through the gun and its getting better slowly.  You can see some wear on the carrier where it contacts the right feed lip of the factory magazines in pic below; also pictured is the wear on the mags.

Full power ammunition vs Winchester white box has no impact on reliability. Currently I can shoot 20 or so rounds before I have a hangup. Thinking I may need to put a few more boxes of ammo thru the gun hopefully things will smooth out. Some interesting wear marks on the hammer.

The forward assist gas port works well and there is no gas directly hitting the shooter. Last round bolt catch works maybe 50% of the time.

I was running KCI (mags deform when fully loaded, metal lips had to be filed). Ive also run the clear ETS which look cool.  In the end I switched to factory Glock only; and ill probably sell the aftermarket mags.

Yeah I run the S3G in my Noveske lower SBR (LWRC 10" upper) and it runs like a champ. I was hoping the cheap RRA trigger would due til after Christmas.  What is dimensionally different on the ALG?  Looks just like polished surface points.

The buffer sanding sounds like a good idea. Strange that I can shoot 20 rounds at a time now before a hangup. I would expect it to occur more frequently. Also I tested it with a standard buffer; no change. Thanks for the info BTW.

Swapped out notched hammer for rounded version.

Well it helps to read the manufacturers website. Thanks to those who suggested the ALG trigger.

Trigger groups:

All QC10 receivers are designed to accept any mil-spec trigger group, and drop-in triggers that match these dimensions typically fit as well. The trigger slot is milled to a width of .312? and will not accept a wider trigger without modification to the lower (not advised) . With the wide variety of trigger groups available in today’s market it would be nearly impossible to test and list every working configuration, so we can only list what we know first-hand. Because of the increased weight of the blowback system, there may be differences in functionality with certain triggers.

Our test guns all use basic mil-spec triggers and lower parts kits, and we know most aftermarket trigger groups function reliably. We do recommend a standard profile hammer. The Geisselle Rapid Fire and SSA triggers are believed to work properly, though issues have been reported with the SSA-E. Rock River Arms triggers are also known to cause issues.

The following triggers are known to work with our bolts.

CMMG

ALG Defense Advanced Combat Trigger

Update:

Replacing the hammer solved the problem.  The QC-10 bolts like the standard profile hammer without the relief cut.

Gun ran without a problem (last round hold open) when using Glock factory mags. Aftermarket mags had a jam or the bolt didn't lock open.

Final configuration:
Tros 5" barrel.
QC-10 9mm bolt.
QC10 Small Lower
Standard mil-spec trigger with rounded hammer.
CMMG 9mm Buffer
Standard buffer spring.