Hey all.  Gonna take a quick second to introduce myself, skip this paragraph to get to the problem.  Names Matt, stuck behind the curtain here in the PRK.  Been lurking on here for several years searching and getting info.  Lots of great information and helpful people on this forum.  You guys helped me build my first AR about 6 or 7 years ago, and using what I learned, I put my first one together without a single problem to date.  That build was on an MGI modular lower.  My intent was to get their promised Glock 9mm magwell and build a new upper at some point.  After like six years of delays and broken promises from MGI, I finally said to hell with them, and decided to just build a dedicated 9mm with a QC10/DDLES lower.

Parts:
QC10 GSF lower
RRA 9mm upper
YHM 16" barrel
JP 9mm bolt (ramped) and SCS Buffer
Milspec carbine tube with MFT stock
ALG ACT trigger (rounded hammer, comes with 4.5 and 6lb springs)
CMMG pin/spring kit
Diamondhead VRS-T handguard
All other parts, controls, etc are std milspec

The issue:

Put everything together, seemed great.  Passed all function checks.  With upper off, push hammer and it locks on the trigger sear, pull trigger it falls.  Hold trigger to the rear, hammer back, it catches on disconnecter; release trigger, hammer clicks down on to the trigger, trigger resets, pull trigger and hammer falls.  Same with upper on using the charging handle, all checks pass.  All springs are in correctly, disconnecter fat side down and all the way in, hammer spring to the rear over trigger pin.

*continued below*

Took it out for a sight in and shake down run.  Almost immediately I start getting failures to set the hammer.  It is following the bolt home.  Ejects the old round and chambers a new one every single time, but about half of the time the hammer isn't resetting.  Also had several double and triple fires.  Was able to get through one whole mag once without having to charge it, but did have a few double fires.  Also had a handfull of light strikes, and once I stopped for inspection, every round when the hammer didnt reset and i had to manually charge had a very light strike on the primers from hammer follow.  This is all with the 4.5lb hammer spring.

Next day, decided to swap triggers out of another AR.  Std milspec trigger, notched.  No surprise, fails the function check; when holding the trigger and charging, the hammer doesn't go down far enough to grab the disconnecter.  No go, not even going to try to fire it like that.

Tried the milspec trigger from the other AR with the ALG hammer and spring from the first run.  Worse performance than the first run.  No light strikes, only once did it double fire, but 100% failure of the hammer to reset.  Essentially its a single shot today.  Went home real pumped *eye roll*.

Only thing I didn't try was the heavier hammer spring, but I don't see that causing the hammer to not reset.  If anything it should help....less resistance the bolt has to overcome to push it back.  If I solve the big problem and am then having an issue with light strikes, figure I'll try that then....one thing at a time, lol.

I'm really thinking its the JP buffer setup causing a slight short stroke.  Theres no slop, bolt is tight to the breach when closed, no rattle what so ever.  Very minimal play between the receivers as well, you'd have to be super anal to even detect it.  SCS is lightly oiled, doesn't bind in any way when compressed.  Bolt moves freely in the upper when assembled, nice and slick.

My only other idea is buying a DPMS hammer and try running it with both triggers.  Im doubtful this'll work though, ALGs hammer is the same exact profile, and as far as I know they don't remove any material from the hammer on the  sear area, or the front for that matter.

Let me know what you guys got, hoping somebody has seen this before.  Beating my head against the wall over here, frustrated as hell.  Thanks in advance for any help and dealing with this long ass post, just trying to get you guys as much info in one shot as I can.

Thanks,
Matt

I would lean towards the hammer. Their are some trigger groups that just don't work for example the geissele 3 gun. It did all the above that you spoke of and a notched hammer is definitely a no go. A ramped bolt is just a little bit higher off the trigger then a standard BCG hence they are a little bit pickier with triggers. I would try a standard hammer. You could also try a another buffer set up like you said, a standard ar buffer set while not optimal should run

Yeah, I knew the notched hammer was no good for it from the start, part of the reason I went with the ALG.  Was just grasping at straws when I threw it in.

Hadn't even really considered swapping for another standard buffer just for a test run, had my mind set that I'd have to buy a heavier 9mm buffer if the JP unit was to blame.  I'll give that a shot, maybe tomorrow if I have time.  If it doesn't help I guess I'll be ordering a milspec DPMS hammer and trigger set up.  Opinion: if I'm going to replace the hammer, would you just go ahead and replace the trigger too to elimate if?  Might as well I guess.  

I guess if it works another rifle just got a slight upgrade to the ALG trigger, lol.

Thanks for the reply mein

ALG works great in mine.
GSF lower, CMMG bolt cut for Grock mags.

Super 3 gun would not reset.

In most of the trigger failures I have sorted out, the disconnector was at fault 9 out of 10 times.  Either through bad timing, or weak springs, the disconnector is a very finicky little item.  If your not familiar with how to check the interaction of the disconnector with the hammer, JP has one of the better videos that I have encountered on this subject.

Had the same sort of issue, turned out it was the ammo.  Just did not like Remmington 9mm.  Played with buffers and all the usual stuff but it just would not run with the Remmington ammo.   Turns out some of the rounds were loaded a little light and would slightly short stroke such that the trigger was not reset..    The only other ammo it doesn't like is ZQI 125 gn, that will stick in the chamber and not extract.

Contact ALG.  They'll make it right.

Looked it up briefly, but never  pursued it.  Consensus seemed to be if it passes the trigger-back cycle function test, goes on to the disconnecter and back on to the trigger as it should, then it should be fine. Who knows though, lol.  I'll look in to it, atleast its something different I can look at.  Thanks.

Probably will.  Just wanted some opinions, really wasnt sure if should be bitchin at ALG for the trigger or JP for the buffer.

I've only ran Speer Lawman and some old Federal sub sonic hp's out of it, don't recall if the HP's had any failure to resets...they all fed though, which was my main concern.  It could be the Lawman is weak, but its a pretty decent boxed ammo.  If it won't cycle that, it probably wont be reliable with purchased reloads, which 99% of what this thing will be eating.  I have some Blazer Brass and some sxt's I'll try next time to get a little more variety, but I'm not gonna be happy with this thing if its going to be picky about ammo.

I have the same lower, trigger, and buffer. My gunsmith had to take a file to the trigger and take a tiny amount off because of the same problem with the trigger not resetting. Works perfectly now.

To the TRIGGER?  I hope your gunsmith has lots of insurance.  That sort of issue would be something ALG would want to know about ASAP, and as I said, they'd make it right.  But now that your parts have been modified, any issues (doubling? worse?) are on you and your gunsmith.

Bought a milspec rounded style hammer today, threw it in with the milspec trigger and disconnecter from the other rifle, and light spring from ALG, the 4.5 pounder.  No joy, same issues.  No light strikes, but several failures to reset, regardless of how I manipulated the trigger.  Proceeded to install a buffer retainer and spring so I could use the buffer out of the other rifle (JP SCS system doesn't require a retainer, never installed it), only to realize in short order that I'm a dumb ass, lol.  The other rifle has an A2/rifle length setup, so obviously the buffer was too long.  Duh.

Here's where it got interesting though.  Went ahead and threw the JP SCS back in WITH the retainer, for shits and giggles.  The retainer holds the JP SCS back just slightly, like a 16th if anything.  Didn't notice any obvious free play of the bolt when in battery, so it should be sound, but I didn't make it a point to try and wiggle it, it was getting dark.  Crazy thing was, it ran significantly better.  Had several failures to reset and one double, but ONLY when taping/releasing the trigger.  If I held the trigger down after firing, then released to (hopefully) reset the trigger, it worked every time.  Went though about 60 rounds this way, switching back and forth between holding or taping the trigger, and the results repeated every time.

I kinda feel like I have a compound problem now.  What KYliberty posted seems to be part of my issue.  You were talking about the actual sear surface of the trigger, right?  Seems like my main problem is with the trigger sear, not disconnecter.  Add to this the slight gap I added between the bolt and buffer, and I think I may have located my problem.

I'm going to try and borrow a carbine length buffer and spring and I'll post back up.  Going to contact JP as well and see if they had any issues like this.  I'm hoping if I ask nicely, they'll let me buy a standard 9mm buffer from them and return it if it doesn't solve my problem, or return the SCS if it does.

I think it was actually part of the hammer, but that is part of the ALG trigger group. Not 100% sure where he took a tiny amount of metal off but it fixed it instantly.

Also, my gunsmith is very talented. He has done many trigger jobs for me as well as a lot of other types of work. Never had the first problem with any work he has ever done for me.

Sorry I don't know exactly what the gunsmith did to fix it. He pulled the trigger out, took a file to it for a couple seconds, popped it back in, and it was perfect.

One more thing... Mine works flawlessly with 147 grain, but with 115 grain it doesn't cycle properly. Since I run mine suppressed, I never found out that 115 doesn't work in it until today when a friend loaded up a mag with it.

My 9mm with the SG3 trigger does the same thing. Will reset sometimes and sometimes the hammer follows the bolt home.  I think the problem the bolt and that it's ramped. I have been told to try an un ramped bolt and if that works that solves your issue.

It's a runner!  Talked to JP, and they said they had a lighter spring for the SCS I could try, and offered to send it to me on warranty.  Spring was labeled as a .308 "red".

Took it out today and it ran like a Swiss watch.  Was still somewhat dirty from previous failed tests.  Ran 66 rounds of Speer Lawman 115gr, a couple hundred LAX 115gr reloads, and 33 old mid 90's Federal subsonic hp's...unsure of grain, but I think they're 147's.  Ate it all up without so much as hiccup.

Thanks everybody for the input, I appreciate it.

Sounds like I might need to get one of lighter springs in case I want to run 115 through mine.

I'm having the same issue and made a thread also on the first page currently trying to fix it.  I have a non-ramped bolt with a notched hammer (which does not bind, I think because I'm using a Spikes 9mm buffer that is longer to shorten bolt travel akin to the stack of quarters trick), but switched out the FCG to rule out that this weekend.

I'm told I need a ramped bolt and a non-notched hammer, but I think my problem is that the hammer is following the bolt home.  I'm thinking it's a bumpfire issue like I'm resetting and releasing the trigger before the bolt returns to battery.  Fires, ejects, strips and loads a fresh round every time but can't fire again because the hammer is up.  Or down, as it were.  If switching bolts solves our problems, I'll switch with you .  Somehow I doubt it.

I know 9mm AR's can have bumpfire issues and I did with a standard carbine buffer when I first built mine but figured the Spikes 9mm buffer would fix that.  Recoil is still mild compared to 223/556 but I suspect I'm being too light on trigger follow-through which I think is what causes this bumpfire.

First, the buffer won't have any impact on whether or not a notched hammer binds.  The hammer will bind on the bolt itself.  

<rant>Notched hammers are an anti-gun "fix" for a non-problem, and nobody needs them, period.</rant>  A rounded hammer will NOT bind, no matter what.  A ramped bolt will smooth the operation of the whole system, but ramped or unramped, "binding" is not your problem.

Your problem may be that your notched hammer may not be pushed far enough back to catch the disconnector because of the notch.  A rounded hammer will either fix that or rule it out.  If a rounded hammer doesn't fix it, there's a problem with your disconnector or trigger.