Not all airguns have the best accuracy with crosman premiers. This is why most shooters with a several airguns will frequently stock a variety of pellets. In general I find that .22 JSB Jumbo's are more accurate than CPs in most of my airguns. Your results are not out of the ordinary.
However crosman does have some fairly loose specs for their barrels. I had one disco barrel shooting 1/8" JSB groups at 15 yards and another disco barrel shooting 5/8" at the same distance on the same gun. After a checking the crown, I noticed that the bore was larger which cause shallower rifling and loose pellets.
I ordered 3 more .22 disco barrels to see if I get another good one. If not I am going to return them and order a L/W choked barrel.
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