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Step 1. wrap some wax paper around a cleaning rod or other dowel and insert into the barrel at the hop-up end.
Step 2. fill the notch with liquid rubber. I had several products to choose from here. Permatex black rubber sealant, polyurethane caulking, and brush-on electrical tape. I went with the liquid tape on this one. These are just what I had on hand. I am sure there are plenty of other options.
It took 5 coatings of the brush on tape to fill the notch so that it was even with the outer diameter of the barrel. I let this set for a few days to fully cure.
Step 3. Remove the wax paper carefully and remove any excess liquid tape. Inspect the inside of the barrel to insure none slipped past the wax paper. I was able to insert a bb which stuck at the notch but fell through the barrel with a gentle nudge. The cured rubber feels about the same as a thin bucking. With gentle pressure I was able push down and simulate the pressure of the nub.
Step 4. turn your bucking inside out and remove the nub. I used a razor to cut it off but sand paper could also work.
Step 5. Turn the bucking back to right side out and slip it on the barrel. From here I was going to re-assemble everything and throw in a H-nub and call it good. But, I decided to try to make a nub that would conform to the outside of the packing and cover as much surface area as possible in order to transfer maximum surface contact to the bb. So I started with a pen cap which fit snugly on the end of the barrel. Using a razor I cut a small section from it.
Step 6. Sand the piece with the barrel as a sanding block so the curve matches.
Step 7. Trim this nub to fit into the hop-up. I wanted to fill the entire nub space. Insert it and re-assemble the hop-up unit.
And with that, re-assemble your hop/barrel assembly and try it out.
