If you do not want to use the tub, purchase 4" PVC cap one end and use that. I had to clear one from the Boy Scout camp. Soaked it for a day and applied 120 PSI from my air compressor. Came out like a bad day after Mexican food :oops:In our Knight Bighorns, we would soak the barrel in the bathtub with dishwashing liquid before scrubbing the barrel.
It might work in your situation making the powder wet.