Does the Stoner bolt lugs have the KAC proprietary rounded design? If so it's not made to work with a standard barrel extension.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek