I just finished a new build of an AR pistol with a 7.5 inch barrel. My issue is when test firing with 2 rounds in a mag it fires both. I have confirmed the fire control group is assembled correctly twice. Springs are correct and parts look good. If it makes any difference it's an Aero Precision upper and lower with their parts kit, AP nickel boron bolt, Ballistic Advantage barrel along with a Shockwave Blade and buffer tube.
I can't seem to find any assembly or part defect with anything. I have checked the reset manually and if I pull the trigger and hold the trigger it catches the hammer when I push it back. To be honest it fires both rounds so fast I can hardly tell it's fired both rounds.

Your disconnector spring is upside down - fat end goes down into the pocket on the trigger.

It's in the right way. I remember working for a couple of minutes to get the large end to fit in the trigger. I do have another one on the way in case this one isn't strong enough.

80% lower?

Aero Precision gen 2 lower.

Check for bolt jarring causing the hammer to drop, from the sear and disconnector.

Clear weapon.

Put on fire, pull trigger, lock bolt to rear, slam bolt home. Pull trigger. Should get another click.

Next time, (use left hand if no BAD lever) pull trigger, lock bolt open while holding trigger. Slam bolt home. Listen for disconnect and then pull trigger for click.

Then try catching the hammer with the disco and seeing if you can pull the hammer free. Possibly try to (pad the edge of the lower if a minor scratch will hurt your feelings) pry the hammer upwards with a punch or small screwdriver.

I've seen it where a disconnector was poorly timed and if the trigger was released just right, the sear wouldn't catch the hammer. Also I've seen where jarring (bolt slamming home) would have the disco let loose. Both were fixed by filing some material from the bottom of the part where it contacts the trigger in front of its pivot pin. This retards the timing so the disconnector releases the hammer later in the release of the trigger.

ETA: in both of those rifles, they worked fine during a typical function check, but live fire would cause the issue. Out of spec parts happen.

Aero Precision has had so many quality issues of late you can probably chalk it up to out of spec parts.

Repost this on Aero'S sponsor subforum and you should hear back from a CS guy named Jeff on there.

WD-40? I saw an AK-47 that had been soaked in WD-40 for long term storage that would double and triple. The WD-40 had turned into a varnish like substance on the trigger works and caused it to malfunction.

A fresh coat of WD followed by an aggressive toothbrush and Gun Scrubber got it running like new.

My original issue has been resolved by changing out the carbine buffer with an H2 buffer.
New action item is occasional failure to eject. Case is extracted but not fully ejected. Any suggestions?

Now it sounds like your buffer's too heavy, causing the rifle to short-stroke. Does the bolt come far enough back to lock open if you only load one into the magazine? Does it pick up a new round?

It's possible that the extractor is dropping the case before it gets out the ejection port.

I have had this problem with MY AR pistols. Most AR pistols are over gassed.

An O ring around the extractor spring can help. But a better solution is to slow the bolt down, either with a heavier buffer or an adjustable gas block.

Disregard-didn't read all posts throughly