You should try the the JSB 15gr pellets. The 18s are great to have on hand as well but you’ll have a slower velocity with them than the lighter 15s. Sometimes that works out to more accuracy though... so it can be a give and take as far as velocity and accuracy goes. Those are two of the best cans of pellets to have on hand to try in any .22 pellet gun. In my experience most often it’s going to prefer one or the other.
Also not to be overlooked is have you cleaned the barrel up? Crosman bores are usually pretty nasty with whatever grease/lubricant is used on them. Cleaning that up will make a big difference. Also make sure your bolt o-ring is in good shape and gets a drop of silicone oil on it every 50-100 shots or so. If that o-ring is damaged you’ll always have inconsistent results.
You are probably already aware of this, but I will write it anyway in case someone else is looking for answers in the future and comes across your post. Your Co2 pistol is very dependent on the temperature, the colder it is where you are shooting, the less power it has. The warmer the better. Each time you shoot the pistol and the co2 is expelled, it makes the gun get colder and colder. So, giving the 2240 time to warm back up a little between shots will give you the most consistent results.
That cold air will also cause the barrel to cool and shrink which changes the actual bore size throughout a string of shots. This is most noticeable in rapid firing co2 guns or full auto co2 BB guns. As a result of the cold, lots of things start happening that negatively affect power and consistency in co2 powered guns.
Obviously I don’t expect that you are rattling through the shots like a Gatling gun with the bolt action 2240, but prolonged shooting sessions will have a cumulative effect on the temperature. For plinking... no big deal, but for precise target shooting, slower is better.