Causes of Bad Accuracy and their Cures.
97% of the times a client brings me a air gun complaining about bad accuracy, it is the stock's screws that are loose. This can be fixed by taking out the screws and using either Loctite or finger nail polish and tightening them. Let the glue cure over night before using the air gun. Then re-sight the air gun again.
Another factor for bad accuracy is a dirty barrel, the barrel might have a build up of lead dust and oil. Clean the barrel with a degreaser, using either a trimmer's line and a cloth or a nylon brush air gun cleaning rod, dry well and give it a light lube of Slick 50 One lube, spray can type. Slick 50 One lube will help prevent rust and lead dust build up. A cleaning rod can be made by cutting a sharp edge on one point of a piece of a trimmer's nylon line for pushing a small lint free cloth on the line, the other end of the line must be melted down flat so that the cleaning cloth does not come off. Cut the trimmer's line one foot longer than the barrel.
At times bad accuracy is caused by a miss-shaped crown of the barrel. An uneven hole of the crown will cause one side of the pellet to be released before the other side of the pellet, causing the pellet to tumble in flight. Exam the barrel's crown, if it is not evenly shaped, it will have to be re-crowned. This is done using a round headed bolt with out a screw slot with a variable speed hand drill and rubbing/ polishing compound. Insert the bolt in the hand drill and using the round head evenly at a low speed slowly polish down the hole of crown until it is even using a rubbing/polishing compound. I even heard of people using Colgate's tooth paste as a polishing compound for this.
Bad accuracy in a break barrel can be cause by a loose barrel pivot point, causing the barrel to cock in a different position each time it is cocked. Exam the pivot point of the barrel, if it moves a little to the sides it will have to be corrected. At times this can be corrected by tighten the pivot point of the barrel. If he still moves a little after tightening, it will have to be shimmed on one side with plastic to remove the movement. The plastic flat side of a empty car's motor oil bottle, black in color can be cut out to the shape of the pivot joint's base. Place the plastic on the side of the pivot joint that favors your sighting. This shim should correct the movement and will not be notice because it is black in color, it should last for years.
Bad accuracy in a scope is usually caused by scope creep. This can be correct in a few different ways. One using a scope stop. If this doesn't stop the scope creep, use loctite on the scope stop's clamps and on the scope's clamp and let it dry over night. Sometimes using picture hanging frame double sided tape works. I used this double sided tape on a light Chinese 4x15 scope on a B-21 and the client says it is holding up, no complaints about scope creep.
Over oiling the piston and the pressure chamber can cause bad accuracy. This is due to the combustion/dieseling created by the oil, this can cause jumps in speeds (fps) of the pellet. Shoot the air gun until the dieseling settles for better accuracy.
Cheap pellets, using poor quality pellets can cause bad accuracy. Try different types of pellet brands to see which ones work well with your air guns. I personally use Crosman domed or pointed pellet with great results in my Chinese air guns.
Holding a air gun too tight can also cause bad accuracy. Hold the air gun loosely but snug when shooting .
Pushing a trigger instead of squeezing the trigger can cause bad accuracy. Take a deep breath and squeeze the trigger slowly while you concentrate on the sights and the target.
Dropping the air gun fast after pulling the trigger can cause bad accuracy. Take in mind that a air gun is not a real high power gun powder weapon and time is needed for the pellet to leave the barrel. Keep the air gun sights in place on the target until the pellet hits the target.