The transfer seal in the 22xx and 13xx series of Crosman airguns has always been a weak point. The trouble with it is that there is a 1/4 inch seat on the valve, a 1/4 inch hole the tube, a 1/4 inch opening on the breech, but the barrel seat is 3/16. To solve this a 1/4 inch seat is needed on the barrel, to allow the use of 1/4 inch tubing to be used as a transfer seal.
Clamp the barrel in a drill press vise. To ensure that the barrel in squarely clamped, insert a drill bit into the port to use as a guide. The use of some sort of padding on the vise is recommended so that the barrel does not get scratched. If you are modifying a barrel which has the flat spot on the end for the front sight, make sure that the flat spot is perfectly level. If the flat spot is not level, the front sight will be crooked.
The barrel is now ready to be drilled with a 1/4 inch drill bit. Do not drill too deep. Keep an eye on the thickness of the port where it enters the barrel's bore. You need to leave about 1/32" of material at the bottom of the transfer port.
Now that the barrel's port is drilled out to 1/4", the bit needs to be changed to the 1/4 inch router bit. Do not remove the barrel from the vise until the machining is complete. The bit I use is designed for the Dremel rotary tool. There may be better bits to use, such as 1/4" cutting bits for milling machines, as long as it will cut a flat bottom hole.
Line up the bit with the the port's opening and cut only enough material to create a flat edge about 3/64" wide around the bottom of the port for the tubing to seat against. Use plenty of cutting oil and enough speed to prevent chattering of the bit when cutting the seat. Once the seat is cut in the barrel's port, use a small file or emery cloth to remove any burrs from around the edge of the opening. The barrel can now be removed from the vise and cleaned in preparation for bluing.
Degrease the barrel with alcohol, or a fluorocarbon spray such as brake parts cleaner, and with a clean cotton swab apply the bluing agent. Allow the agent to work for a moment or as per instructions for the type of agent used. Wipe the area dry with a clean cloth, degrease and repeat until the bluing is dark enough.
Now that the barrel is complete, the vinyl tubing needs to be cut to the proper length. Start with a piece of tubing a 1/4" long, and test fit the barrel with breech to the tube and valve. You need to have a small gap at the front of the breech, only about 0.010" is needed, so that the vinyl tubing is compressed slightly when the breech is tightened down.Important