Martin - I sincerely hope that fiberglass rod is coated with something. Using a bare fiberglass rod in a muzzleloader, or anyting else, is like loading and cleaning with a long round file.

Do a test, take a piece of steel and rub the rod on it forward and back about 20 times each way. A fiberglass rod will cut a groove in the steel like a very fine file will, albeit with less strokes.

I used to use one for my tight twist custom .50 barrel when experimenting with various tight mechanically fitting bullets. I had to re-crown the barrel every 200 shots (3 to 4 range sessons) due to the grooving around the muzzle and loss of accuracy. Cutting 1/4" off the muzzle and re-crowning restored accuracy every time. It was a MOA rifle(ie: consistant 1" at 100 yards) so the drop in accuracy was easily seen. I'd cut and re-crown when it opened to 1 3/4".

With my current steel 'range rods' for the small bores, I have collars to protect the bore, made from rimmed ctg. cases with the centre of the base bored out to fit the rod. The rods are long and therefore allow easy use of the collars. The hole is rod-sized with the jags slightly larger to keep it on the rod. The rim stops the collar at the muzzle. i use 1/4" aircraft stainless for those rods, one for the 42" .45 and the other, a bit shorter for the .36", .40.

The rod for my .69 is nylon.