Think of an oz of leather being 1/64 of an inch. 8oz = 8/64" = 1/8". I've never done a stock cover with cartridge loops, but I try not to tool or stamp on anything thinner than 6oz. I definitely wouldn't go any thinner than 10 oz on a sling as veg tan cowhide does have a tendency to stretch the thinner it is. I'm fairly new to this as well, mainly doing holsters, belts, and some fire department related strap goods, but it sounds like you're on the right track.