The size of barrel I use for my launchers is 2". The packing I use is cheap and creates a perfect seal (if made properly), 2 ways to make, Materials #1:
Solid round, foam noodles, slightly larger than the internal dia of barrel, snug fit.
Cut noodle slightly longer that internal dia of barrel (keeps packing from turing in the barrel and air from flowing past)
#2 If noodles cannot be found for a snug fit in the barrel, Materials:
Solid round, foam noodles, the closer to the internal dia the better, an easy slide from one end to the other (may need to cut)
Roll of foam (usually blue or grey) it is used in construction to insulate the wood from the concrete foundation, usually found at hardware store.
Zip ties or glue
The noodle needs to fit loosly within the barrel and cut slightly longer than the internal dia. Cut the thin foam 1/4" larger than the internal dia, center and zip tie or glue to the noodle section.
These I have found are excellent for creating the perfect seal for Co2 and compressed air launchers.
