A friend of mine has the same break barrel air rifle and we have been using Air Arms Diablo field pellets for the longest time. Are only about 8 grams or so, but under 50 yards if placed correctly the hit with a nice velocity. However, their accuracy can be a bit suspect in windy and high humidity conditions.
This.
I use them almost exclusively for squirrel hunting, they work better than any of the more expensive pellets ive used. Even the nice flashy gamo pellets have nothing on these.... Deadly accurate and very hard hitting. Accuracy enough that i can repeatably knock the cherry off a cigarette at 40 to 50 yards with my crosman phantom, never had to shoot a squirrel more than once and for $2 a tin, you just cant beat the price.