To add to what Mapsjanhere said, Brownells Steel Bed is all micronized stainless filler. Bisonite, which was preferred by service rifle armorers for years, also lacks fiber. Epoxy resins often have plasticizers in them these days to prevent cracking rather than depending on fiber. West Systems G-flex epoxy is a prime example.
A good rule of thumb with any of them is just not to let them get thinner than about 0.005" for best strength. That's a second reason (in addition to grip) that you inlet the stock for them rather than just smearing them on the surface. The main plus to the steel fillers, in addition to ruggedness, is to bring the linear temperature expansion of the cured epoxy mass closer to that of the gun steel to reduce differential shape change with temperature.
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