"Bull Denim": A 3x1 twill weave piece dyed fabric, made from coarse yarns. Weights can vary from 9 ozs/sq yard up to the standard 14 ozs/sq yard. It's basically a denim without indigo!

The softness of the denim I use comes from the brushed finish, not just the weight of the denim.

The "heavyweight" bull denims at Denver Fabrics show a weight between 11 and 13 oz. That will definitely make a sturdy kilt. It will also be a challenge to sew at points where you have more than 6 layers to go through. But it's quite possible.