My friend just finished his first AR15 build the other day. I tell him to give me a call after he takes it out on its first trip.

He calls me later and tells me it must have a 3 round burst installed (used lower came complete). I start laughing and tell him that is kind of doubtful. But out of curiosity I ask if there are any extra holes in the lower (I told him where to look)...and he replies negative. Hmmmm is all I can say. Then in a moment of brilliance I ask, "Can I come over and take a look at it?"

I take a look at it and every thing looks fine, but sure as heck...every time you pull the trigger it will fire three rounds. Kind of neat....but not in the BATFE's eyes

A quick look compared to my RRA internals are for conversations sake "identical", but something is screwed up for sure! I have not disassembled his lower yet, because quite simply....I have never seen this happen before.
There are no extra parts installed in the lower, and the parts inside all look relatively new.

Can somebody tell me what is wrong with his gun? (I know..."nothing", or "what’s wrong with that" will pop up - but seriously is this dangerous, and what is causing it?

I guess I should just strip it part by part & compare to my components and try to figure out what is wrong?

Everything looks as it should from above.

Any ideas?

I really don't know, but my SKS Sporter has thrown a few double shots in the past.

Check headspace.

Come on man, people pay big money too make their guns do that!

Check and make sure you put the spring in the disconnector and that it moves freely in the trigger.

If the lower came already assembled, the builder may have simply used crappy parts. I'd replace the trigger group with good parts (I prefer Bushmaster). Also, I would NOT continue firing the weapon this way because if the hammer is following the carrier you might wind up firing a round while the bolt is out of battery and that would be BAD news (kaboom!).

If the trigger pin hole is drilled to low that can happen also. Might want to have the distance from the top of the lower down to the trigger pin hole checked if none of the normal causes are the problem.

It is very dangerous to have a full auto weapon that doesn't rely on just blowback for its action without having an auto sear. The auto sear is designed to prevent the hammer from falling on the firing pin if the bolt doesn't fully seat. So the gun could blow up without it in an AR-15 because it can't handle the full force of the round, thus the gas system. I suggest checking the lower. Take the upper off the gun. Then cock the hammer. Pull the trigger, keep the trigger back, and then cock the hammer again. The disconnect should catch the hammer, and then when you release the trigger the disconnect should release, and then the trigger should catch the hammer. So if it’s firing 3 round bursts the disconnect won’t catch the hammer until it has fallen 3 times. You may also want to consider some new parts for your lower. Such as a disconnect, trigger, and hammer. I’ve seen an M-16 blow up before; it is not a pretty sight. The user lived, but had to go to the hospital. As for the gun, it was toast. Be careful and stay safe out there.