PLug the the breech end with a silicone plug and lock the bolt in place. Filll the barrel with non sudsing ammonia/ blue kind and stick in a ss cleaning rod. with electricla tape at the end to insulate it and various bands of tape along the rod to prevent the rod from shorting. Attach two wires to a single d-cell flashlight battery and attach the positive wire to the barrel and the negative to the rod and watch her bubble and foam. After five minutes remove the rod and clean the carbon and blue buildup and then reinsert and let sit for a 1/2 hour max and check again. Wouldn't try it on a custom gun but tried it on some trash beaters and I was thoroughly impressed. A mosin nagant junker is definitely a gun I would try it on.( did it on a old savage and a old .303 brit) just don't leave for long periods unattended and do it outside in a garage. FOr barrel block I use a silicon caulk gun and shoot out a little hersheys kiss shapped peice of silicone caulk and let it harden and insert in breech and lock the bolt in place to seal it up. get the rod at a hobby shop or use a model rocket guide rod like I did first time. This is experiemental so beware and use at own risk. Clean up rifle and barrel extremely well after use. No need to warn me about the potential harm of electrolysis from a 1/2 hour at most of a single d cell battery. negligible and not a worry on the guns i used it on as they wer ein bad shape to begin with. Hopefully it will be of some interest to someone. take care.
( got a more complicated versions and recipe from the net with ph balanced ammonia solution and radio shack parts but this lazy version works for me on the two I used it on and real cheap)