The easiest way to remove that plastic fowling is as follows:
Heat up some hot soapy (dish detergent) water on the stove
While waiting for the water to heat, plug the muzzle end of each barrel with a tightly wadded paper towel.
When water is hot, fill both tubes with your hot soapy water.
Let stand for a couple of minutes, then remove plugs; crud will slide out with liquid.
The heated barrels will quickly evaporate any moisture; then the barrels are wiped down and cleaned barrels are ready for the next round.
