The way to avoid the mess with Wipeout is wrap the straw with masking tape to about the neck dia of the chamber, insert the straw into the chamber, give it a half second burst, let it foam for a bit, another half second burst and wait until it stops foaming before removing the straw. If you use short bursts and wait for it stop foaming up you'll reduce the excess that foams out the muzzle or back into the action.
Then set the rifle with the muzzle pointing slightly down to avoid having it run back into the action. I use a cardboard box with V cut-outs and rag under the muzzle to catch the drips.
I had used it in the past, but gave up on it because it's so messy. Your way sounds like it may work without so much mess to clean up. I've have to give it another try.
DF
