Not speaking to the "Old West" holsters, but to the more modern IWB and OWB most makers are going to use 7-8 oz leather. One member, Katsass, uses two pieces of 4/5 oz glued flesh to flesh to make his 8oz leather for the holster and it makes the holster very rigid. I can attest to it as for one of the styles I have made I used 4/5 oz glued together and it is very rigid. Even on horse hide which is more dense than cowhide I have used 6/7 oz for IWB and have not had any issues with the mouth of the holster wanting to totally collapse. It all has to do with the right methods and reading in this forum can fill you in on most of it. The rest is the hands on learning curve.