[lugerholsterrepair]

Chris..It looks very much like hemp rope. If you were to unravel some natural hemp rope and cut it about three inches long this would approximate the material well enough for your vidio purposes I suspect. Just wrap enough of it to fit down the barrel.
Flax is very similar, both being a natural plant fiber. There have been several photo's of this stuff on the Forum over the years.

Also, I don't want to spoil your shooting fun but couldn't you spray some dry grey graphite on the pistol to approximate carbon build up for vidio purposes?
Jerry Burney