Actually, the duct seal I was thinking of is dark grey and comes in 1 lb. blocks, you can find it in the electrical section. It's kind of like a stretchy, sticky, rubbery modeling clay, and it never really dries out. I had a block in my gun for awhile, took it out because I was doing something, and never got around to putting it back in. I need to put it back, it'll be interesting to see if it makes a difference with the gas ram. Later.
Dave