Failure to go into battery

[A5BLASTER]

Do not take anything apart until you make sure your chamber isn't short and your jamming.

Drop a round of ppu120 in the chamber, muzzle down bcg locked back. Make sure there is no mag in the mag well.

Hit the bolt release and let the bcg slam home then see how it acts when trying to extract the round.

Come back after doing that and update us and we can go from there on helping you figure it out.