As it's a Prosport I would ease of the heavier pellets until the seals have bedded in. With the tight heavy pellets it sounds like the piston is coming to the end of its stroke as the pellet is exiting the barrel.
You should put a tin of normal AA field 16g down it and then see.
Give the barrel a good clean before you do. Don't be tempted to try any others until the full tin has gone,as other pellets will have a different hardness of lead that could foul