Quote Originally Posted by piggy589 View Post
Thanks bud,the seal on the metal piston head came out of our air-con. selection box and is a nice sliding fit.

As you suggest it might be an idea to test a range of pellets to find one that fits snugly in the breech plug.
Other than the seal, if it is still a problem then there is a chance the tap is misaligned - shoot into some loosley packed cotton wad or something that wont deform the pellets and see if they are going into the bore squarely. Check that the piston rod is straight. GOod luck !