It was probably not strictly a "finish", but a byproduct of manufacturing the Damascus barrel, made by twisting strands of iron together around a mandrel and hammer welding them together.
Knives with damascus blades are made by starting with a long/narrow strand of steel or iron, folding it, and hammer welding it until it's a solid again, folding/welding, folding/welding, etc - until up to over 100 layers are achieved before the final shaping & finishing.
As far as "tiger striping", that is a finish used on Maple gunstock wood, sometimes referred to as "Suigi" (or "Sugi") that results in an alternating light/dark striped uneven pattern, before the final topcoat/finish was applied.
.