So I am finishing up both my CMMG RDB build and a standard AR9 build. I took both out to test yesterday and had a strange issue with the standard AR9.

The trigger doesn't reset after taking a shot. The next round chambers, and after I pull the charging handle back and release, the trigger resets fine. I tested it inside with no rounds and found the same. If I hold the trigger down and pull the charging handle, the trigger remains in the rear position. If just pull the charging handle back with the trigger loose, it works fine and all is well. I am lost, I just can't figure it out.

Specs:

LaRue MBT2S
AIM Surplus AR9 Bolt
Standard Aero upper and lower
Spikes T2 buffer and SI Flat Wire spring

Thanks for any info!

Double check the orientation of the hammer and trigger springs

Double and triple checked, I found no issues.

The MBT my not be compatible with your 9mm blowback

Is it a trigger issue or a bolt issue? I tried it with my PSA bolt and it has the same issue.

However, I just ran the trigger 20 or so times(not letting the hammer fall of course) and it's working good now. I tried it in the upper 20-25 times, not a hiccup.

I will test at the range tomorrow.


EDIT: I forgot I swapped the PSA bolt in. The PSA bolt did not work when first swapped in but after the above mentioned running of the trigger it worked. The AIM surplus bolt(which is the bolt that I need to work in the end) is still not working.

I had this issue with weak ammo.  Once I put good quality ammo in my 9mm SBR, it runs like a top.  Just a thought.

Compatibility between the two.  I've also had issues with different trigger/bolt combinations.  9mm bolts are not all made to the same specifications, and not necessarily the same specifications as 5.56 bolts.  Triggers, other than the standard milspec, can be hit-or-miss unless the manufacturer states that it works with 9mm.

Someone over at Aim Surplus is going to test one of these bolts with his MBT2S on Tuesday.

That should probably get it sorted out exactly what it is, either a one off out of spec bolt/trigger or just straight up incompatibility.

Thanks!

check your disconnector.

I am following this as I have a similar but less severe problem.

I have an Elftman trigger in my PSA AR9.  About once in 30-50 rounds while firing I will hit a hard or dead trigger. It usually happens during a competition when I don't have time to troubleshoot it.

I can work the charging handle, an untouched round ejects, hammer is cocked and it is good to go from there.
I can never duplicate it at home. I have checked the disconnector and it seems to work exactly as it should.

The ELF trigger has such a short overtravel/reset I wonder if that is not part of the problem that the trigger is not reset.

I thought the bolt was not being held back but, now I am wondering, as I usually hold the trigger back and fire from reset if it is just not letting the trigger and disconnector work properly to reset and fire?

As stated above, known issue (and if I remember correctly quite a few no-go's with the MBT).  Some bolts an triggers dont play well together in the 9mm's.  

Lots of info in this thread.

Style of ramp and top of hammer interface..

Manual charge you generally pulling bolt back farther to allow hammer to tip enough to catch disconnect..  Make sure you using a M16 hammer that is up to spec for height..

Also cut off excess material from tail of hammer as seen in pic

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M16 hammer has excess material beyond lug, and with 9mm bolt geometry,, it can get push back and hit disconnect and load the hammer (think broken hammer pin and oblong pin hole)...

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