Without that plastic retainer the motors can sometimes back out and their gears won't touch. If you order the VS units, they'll come with them, so do the HLs.

The breach is the part of the gun that the bb falls into when the nozzle retracts. Inside the breach is a rubber bushing that holds the bb in place and keeps it from sliding down the barrel. The thin side should face the bb, the thick side against the barrel. The barrel also goes into the breach. Some of the heng long screws that secure the mantlet to the airsoft unit are a little long and touch the breach, especially the left side. If yours has the indentation from the screw and is not cracked, cut the screw down so it won't touch it. If the breach is broken you can try to fix it or order a new one. If you install the VS breech in the Heng Long housings you need to modify the breach and housings a little for trouble free feeding. Shenloco on MTG and Panzer forums has posted some great pictures.