i dont use that little tube that comes with it.
What I do is, open the floor plate, stuff a bunch of paper towel in there, then give the can a shot into the end of the barrel. Quickly put my finger over it so it doesnt come out, and watch the foam expand its way up the inside of the barrel until almost at the chamber. Then remove your finger from the end of the barrel.
I've talked to a few other people and the other way one person does it is; stand the gun on the muzzle in a plastic bucket. Using the tube, spray foam into the gun from the chamber side. Apparently all the crud comes out the bottom of the barrel and into the plastic bucket.
I've never tried this method since I dont want to chance my gun falling over and being damaged.