Choosing the correct pellet is very important when looking for the most accuracy, or power, or damage a pellet can do when hunting, target shooting, or plinking. Unfortunately every barrel of every gun is a little bit different and this will take some trial and error to see what your air gun likes the best. There are some brands such as H & N that not only offer different grain and pellet types, but also different head sizes of the same type. The head of the pellet is one of the parts of the pellet to be effected by the rifling in the barrel when the skirt is expanded and pushed out of the gun. In some cases you may have to have an average or undersized pellet. Then in others you may need a slightly over sized pellet. In .177 caliber for example. The standard head size is 4.5mm So depending on the gun you may see better performance with a 4.49mm or a 4.51mm head size.
You will have to do some experimenting to see what works best for your air gun but finding the right pellet head size can make a huge impact to how well your gun performs for you!