The SG 552-1 Commando is an airsoft gun replica of an actual SIG SG 550 Commando assault rifle.
The safety switch isn't working.
If the safety is not working properly, then the safety is most likely broken. The best way to check whether or not the safety works is to put the gun on safety and try firing the gun. If it still fires, then the safety is broken. If that is the case then you will need to open up the gun to get to the safety switch and then replace it.
The airsoft gun is not firing.
If the gun does not fire, there is a chance that the gearbox is jammed. You can first check and make sure that there is not a BB jammed in the barrel. If there is a BB in the loading chamber, then the nozzle can be jammed and this will cause the gearbox to be unable to turn. If this is the case, a cleaning rod can be used to push out the jammed BB's. If there are still BB's jammed inside the gun after using the cleaning rod, then it may be necessary to disassemble the gun and remove the BB's.
If the gun is not firing, then it is possible that the spring is broken. If the gun is cocked back and the trigger is pulled but nothing is fired out of the gun, then the spring is most likely broken. If that is the case then you will need to replace the spring.
The BB will not load into the chamber.
If the BB's are not loading into the gun, then there is a possibility that the tappet plate is broken. The best way to check if the tappet plate is broken is to see if the nozzle moves when the gun is fired. If it does not move, then it is highly likely that the tappet plate is broken. A broken tappet plate would still push air out but since the nozzle doesn't cycle, BB's are never actually loaded. If it turns out that the tappet plate is indeed broken, then you would need to replace the tappet plate.
The head joint gets stuck when cocking the gun.
If the head joint is stuck, it's usually because BB's are jammed in the receiver. To push the BB's out, use the reverse end of a cleaning rod.
Nothing comes out when trigger is pulled.
If the gun is fired and no sound is made and nothing is fired, then the trigger is most likely broken. To fix this, you would need to disassemble the gun and replace the trigger.
If the trigger doesn't work, then it is possible that there are shattered BB's inside of the barrel. In order to clean the BB shards out, you will be required to take the barrel out for cleaning.
The BB's are not shooting straight.
If your gun is not shooting straight, then there might be dirt or broken BB's inside of the barrel. If this is the case, then you can use a cleaning rod to run through the barrel. If the barrel is still not clean after that, then you will need to take the barrel out to clean it more thoroughly.
If your BB's are shooting higher than usual, then the hop-up needs to be adjusted. If the shots curving down before they should, increase the hop-up slowly until the BB's are flying straight and level. For increased range, you can also increase the hop-up slightly more so that the BB's arc up and then down.