These guns are notorious for poorly sealing breeches. Look at the pics. Barrel contact, poorly cut chamfering on the barrel, leaking breech seals, all of which will reduce the velocity on an already low powered gun. If you have barrel contact. First resurface the valve body face, then rechamfer the barrel to get rid of the burrs. Install a shim under the oring and set the barrel gap. The gun will easily shoot to factory specs if the breech is sealing and the pellet skirt sits on a polished barrel chamfer. RWS R10 4.49 head size pellets are what I recommend you use in these guns. These shims were made to fit my FAS 6004 barrel. May also fit the Sig Sauer ASP Super Target as it used the same power plant.  Manufacturing tolerances on barrels may require you use a needle file to adjust the shim washer to fit your barrel. If you have dings or burrs on your barrel you will need to clean them up. Pictures show problems, corrections and some mods I have done on my guns. 

TEN stainless shims are included. OD-9.525mm, ID-6.5786mm, Thickness-.2032mm

Nothing else in the pictures are included.