First here are the rifle particulars
Colt lower with Geissele trigger, "H" buffer, fired thousands of rounds without issues

Upper - new professionally built, fired less than 200 rounds
New Colt 6920 bolt and carrier assembly
Used Colt upper with new Ranier Ultramatch 10.5" barrel
New Ranier gas tube and gas block
Suppressed with AAC M4-2000

Ammo - Factory M193

I had what I thought were bump fires (2 rounds back to back), say once every mag or two. In addition once every mag or so the rifle would not fire, the bolt was all the way forward, had a live round in the chamber but you could not pull the trigger.

I racked the slide and ejected the round and was ready to go again. I took a closer look at the ejected rounds after the failures and each had a light primer strike, not from pulling the trigger but from the firing pin striking the primer as the carrier went forward. Starting to wonder if the "bumpfires" were actually the firing pin striking the primer without pulling the trigger or the trigger not re-setting and striking the firing pin as the carrier goes forward.

Any ideas what may be ocurring and how to fix? I don't work on my own rifles so if it's anything other than swapping a part I will be taking it to the gunsmith.

Left round fired, right round live and ejected after malfunction.

That dimple happens in every AR, it is caused by the floating firing pin and is not a concern. I'll bet my next paycheck that it has nothing to do with your malfunction.

When you say "professionally built," who built it? I think it's safe to say that in the collective arfcom experience, claiming to be a "professional" means as many things as claiming to be an "artist" does (i.e. they might not know shit).

I kind of wonder if your double firing may be from inadvertently resetting and pulling the trigger during recoil. I've seen a girl do that when shooting a rifle that had a SSA-E trigger. I figured that she was holding it so gently and pulling the trigger so softly that it was causing her to double tap immediately after the first round went off. I'm not knocking the way you're holding it, just suggesting that you might be pulling the trigger twice and not realizing it?

Also, when it doesn't go off, is the hammer cocked? Maybe it's short stroking and not cocking the hammer? Although this would be hard to do if it's chambering a fresh round but not cocking the hammer. If it's cocking it when this happens, does it feel like pulling the trigger with the safety on?

As many details like that would help

ETA I re-read this and wanted to make clear that I was not calling you a girl. I meant that a shooter, who happened to be a girl, was inadvertently double tapping.

 THIS 100% ...... I have doubled several rifles  by gently pulling a match trigger while the rifle sat in a rest with only my shoulder touching the gun... Good luck. WD

The disconnector you must fix.

"Hammer following" because the disconnector is failing to catch. When the trigger is pulled, it is no longer able to catch the hammer sear surface, the disconnector is supposed to catch the hammer. As the trigger moves forward, the disconnector releases and allows the hammer/trigger sear surfaces to catch.

The light firing pin strikes are caused by the floating firing pin, striking forward as the bolt goes forward and rotates into battery.

You may have gotten the rapid firing due to the hammer being released just fast enough to fall as the bolt went into battery. When the hammer slightly slower and was allowed to fall prior to the bolt being in battery, you got the fail to fire and light primer strike,

Concur with Will.

My kids have done the same, bump fire a few rounds, and on the third or so nothing. With them, the rifle wasnt going all the way into battery.

If you have a good hold on your rifle and it bumpfires, you may have hammer follow. Swap out your disconnect and see if that solves the problem.

Topic Moved

That is not from the floating pin hitting a round being chamberd. That's way deeper then the "ping" you normally see.

I agree with dano that you have hammer follow caused by a broken disconnector.

When grinding the JP trigger, they have a video of how to test this. You fully depress the trigger with hammer cocked, dry firing it. Don't let off the trigger. Slow let off the trigger and listen for the disconnector(I guess?) to release. It should do so at the pinnacle of the release stage, so on a single stage trigger, it shouldn't let go until the trigger is almost back to its default cocked position. If it lets go before then, you have to adjust the screws on that trigger, or you may have ground too much off.

In practice, incorrectly set, this will cause the gun to double tap.

Maybe someone can find that JP Enterprises video, it explains it very well.

Long and short, there is a way to audible and accurately check the disconnector setting through dry firing. I just am no good at explaining it.

Update from today's range session

Update from today's range session

I agree with Dano that your disco spring is probably installed upside down. Willing to bet money on it.

Disconnector appears to work properly when doing a function check.

Looking thru the archives it appears the upper may be overgassed when running suppressed and I may need a heavier buffer or buffer spring? The reason I suspect this is the rifle works fine unsuppressed, no trigger issues or malfunctions.

I think I will consult the builder as well to get their input.

Talked to the builder today who suggested it may be overgassed causing the bolt to bounce. Installed a heavier 9mm buffer and it fixed the problem, fired 180 rounds without a malfunction.

I will take your advice and have the disconnector thoroughly inspected to take care of any issues.

Thanks to everyone for all the input.

Did it end up being the disconnecter spring? Just curious if we were right lol