Barrel-stub head-space gauge, and for new BBL, perfect seating depth gauge: this, or, for when ya have to, "chase the lands", the method displayed by Alex.
P.S. have received a few questions: this set of pics displays my typical/initial 30BR/30Anything set-up.
Hypothetically, it would be 0.030" of JAM: however, with the FL die neck/bushing diameter 0.004" smaller than the loaded neck-diameter, upon lock-up the bullet wedges into the lands, and sets-back in the case neck - a "jam", or, "soft-seat". Having done this, for future reference,I record the seater-stem length & reamer identity in the top of the L.E. Wilson die box. A different bullet configuration, or, a bullet from a different point die, will, of course, require a different stem length.
As previously pointed out, by others, and worth repeating, are the critical points:
1) working from a "Jam" leaves but one way to go;
2) precise reference for future use with a particular bullet. or, any bullet from the same point-die.
Note: Tough the reamer used to make the gizmo (neck-length/trim gauge) in the last pic has a LONG free-bore, a close look reveals the beginnings of the lands. These gauges are all made with barrel
"drop-offs". Ideally, the stub length will be long enough that the bullet nose is completely contained, thus accommodating precise use of the caliper.