Getting in line with Paulosantos for his SPC II reamer is an inexpensive and easy way to alleviate some pressures from your tight M1S barrels' SAAMI chamber. You could work up loads like David did watching your brass, or stick with the commercial off the shelf 6.8 ammo.
We always tell shooters who already have M1S barrels the SPC II chamber reamer will help, but the over fast twist still limits you to off the shelf ammo, or careful reloads. Lowering pressures some definitely is easier on your rifle's mechanicals, especially your bolt.
For those considering M1S barrels or uppers, we tell them to look at other alternatives that have the SPC II or DMR chambers, and 1:11 or 1:12 twist. There are many available that are great values. These alternatives to M1S allow use of SSA's Tactical loads, warmer hand loads, and some of the newer higher performance 6.8 ammo that is in the works.
Some of you who have 6.8 uppers already, can just change out barrels at some point since you already have everything else. You can buy Xtreme barrels and Bison barrels for example.