Second round needs force to come out then third round goes bang. Ideas?

I so much want to be the hero here!

The gun is a 10.5 sbr, unknown mfg but I've seen it run before.

My thoughts are: chamber cools down enough to allow seating of the third round due to the time spent removing the second round. The chamber has been cleaned with a chamber brush.

I asked if they tried a mag with an odd number of rounds(13 or 15 as opposed to 30 or 28). I don't know why it would make a difference but it's the best I can do from my chair at my desk.

All the ammo is factory.

Thanks

The extractor has the helper kit(o ring)installed.
Just read this in another post so I figured I'd disclose.

Has the action been cleaned?

After that, does it have an adjustable gas block? If so, is it still functional, as in can it be adjusted to allow a little more gas?

Buffer spring still good? Might have become too weak to drive the bolt into battery.

Is the trigger resetting?

Will the bolt lock back after firing the first (and only) in the mag?

It could be any of the things JPK suggested. I had one do like that Mike and I replaced the gas tube never had another problem.

JPK, the action has been cleaned, non adjustable gas block, buffer spring has been swapped with functioning carbine, trigger resets, bolt locks back.

My buddy(not the owner)thinks the chamber is too tight. He keeps talking the R word but I told him at that point it becomes unreturnable/his rifle.

10mmillie, gas tube was replaced as well.

Thanks for the ideas folks.

As far as seeing it run, I was informed that was another one; he has three or four. Poor him, at this point. I vote send it back.

After swapping parts amongst three sbrs my buddy came up with a faulty lpk. Also noted was the walking out of the trigger pin, possibly due to the spring being above the pin instead of below. The trigger pin didn't have the notches for the straight part of the spring to ride on, keeping it from moving out.

During a lull in the firing he heard the trigger reset, after the empty was ejected.