I dont know if this procedure is the right way to do it but we have done it more than a few times.
remove barrel extension and remove .001" of metal from the step where the barrel extension rest and also from the where the bolt lug are to touch thereby moving the barrel extension forward by .001" yet maintaining the void for the bolt lugs to fit in.
if you still dont get the right fit, repeat the procedure again.
I have not yet made the 5.56 mm field guage cause I figured that if we will be fitting bolts this way then I'll just have to use the .223 field guage and get a better fit.

