Just starting to reload.... having problem measuring how far to seat bullet in case...

Using once fired .308 Fed GMM brass, in Mod 12FV, using 168g Sierra Match bullets.

Tried to measure throat length by making up dummy round using once fired brass and slightly ovaling neck so it will hold .308 round lightly.

Placed dummy round in breech and chambered round, slowly eject round without releasing firing pin.

Measuring OAL varies from 2.815" - 2.887"! OAL of Factory loads of GMM meas 2.800"-2.805".

What could be causing variance in measurements? Does floating bolthead have anything to do with this?