Re: seating depth by oal guage or chamber method?
I use the "Ranging Rod" method i read about in a handloaders digest many years ago. This consists of a 30" length of 5mm brass rod ( for 22c/f and 243, 6mm, etc,7mm for 308) too which i have attached a locking collar approx 1" diameter and 1/2" thick.
Firstly i lift the bolt to cock the firing pin, i then slide the brass rod down the muzzel until it stops against the bolt face. i hold it in place and move the collar to contact the muzzel and look it in place.
With my Vernier i measure the protrusion of the rod from the face of the collar 3 times and record the mean reading.
The same process is then repeated but this time with the bolt removed and i have an assistant wedge my chosen bullet into the lands and hold it there with a suitable piece of dowel. The rod is then slid down the muzzel untill it touches the tip of the bullet, the collar locked and a reading of the protrusion taken and recorded as before.
Subtract the smaller dimension from the larger dimension and this will give you the exact C.O.A.L. for that bullet.
I then make up a dummy round to this exact C.O.A.L., take a bullet comparator reading and use this dummy round to set up my dies.
In my opinion this the most accurate way of finding your C.O.A.L and it may be a bit of a chore to do this with every different bullet you use, but in the quest for accuracy i think it is well worth it.
"I mean't to shoot the pike but the duck got in the way"