[Jimski]

someone obviously dropped this aug on the ground and didn't tell about it.

it's even probably fallen on the butt, and that's why your internal buttplate is broken.I suggest you check the hop hup, the plastic bits that hold the plastic ring (the one closest to the nozzle) might be broken if it fell on the barrel end.
that would cause air leak.

I also suggest you check the magazine bay inside the body (it should come out when you remove the mechbox), it could also be broken.

yes, I dropped mine like that too, but it was mine...

you can remove the barrel unit before putting the mechbox back, it sometimes help, especially if your hop up unit has moved from the shock.