I think folks are missing how a barrel nut pre-fit barrel works.
When you screw the barrel on to the receiver and go-gauge until it stops, and then tighten the barrel nut, the tightening process pulls the barrel away from the receiver (thereby slightly increasing the headspace so it is just above minimum, in theory). The amount it is pulled away from the receiver is determined by the stacked tolerances of the barrel/receiver threads and the barrel/barrel nut threads.
I have had some barrels where the thread tolerances and chamber depth were close enough that it was causing problems chambering SAAMI spec ammo. In this case, a single layer of magic scotch tape (.0024" thick IIRC) on the back of the go-gauge will give you enough space to chamber any SAAMI spec ammo, without giving excessive headspace. Just remember to cut/trim the excess tape from around the go-gauge case head, so as not cause any wrinkles or folds that throw off the thickness...a sharp knife, run perpendicular to the tape will cut/rub through the tape so the excess falls away, leaving only a circle of tape on the gauge case head.
HTH...
BTW, a piece of brass if doable, but you need a new piece of brass every time you try to (lightly) seat the barrel. The barrel has enough mass when being threaded into the receiver to slightly crush the shoulder without you noticing. If you use the same piece of brass for several attempts, the brass will actually be shorter than you want it to be, from being crush ever so slightly each time the chamber butts up against the case shoulder.
