I’m 20thou off the lans at 1.851”. What increment do you use to test seating depth?
I do .003 increments but it really depends on how much you want to mess with this. Test at 200yds. 3rds (maybe) per increment.
If mag length allows it, I'd typically start at .002 off lands, going -.003 increments but until I find a window then find out how large that window is. I also bring a Lee hand press, micrometer seating die, and calipers for this test though (I have no arbor press).
So using your numbers the process would look like - Distance 200yds, 50rds would be more than enough. Any leftover rounds after finding the window will be loaded at your window.
- Load every single round at home with seating die set at 1.850" (they will be +/- .005 to .001 assuming you have good bullets, no need to measure every single rounds unless you're bored).
- Shoot 2 rounds of 1.850" , if the holes touch, shoot 3rd. If the holes don't touch, don't bother shooting the 3rd one at 1.850".
- Take 3rds of 1.850" CBTO rounds put it in the hand press and seat the bullets deeper using micrometer seating die to 1.847"
- Shoot 2 rounds of 1.847" if the bullets touch, shoot the 3rd. If it doesn't, take 1.847" and seat that deeper to 1.844" and seat 2 more at 1.844"
- Test the 1.844", repeating the same sequence in .003" increments until you find a window in which all the bullet holes touch and the point of impact is in the same area.
Lets say -
1.844" all 3 bullet holes touch and POI is 1" high at 12 o'clock from point of aim. 1.841" all 3 holes touch and POI is also 1" high at 12 o'clock, then 1.838" same as 1.844" and 1.841". Now if the 1.835" first two bullet holes touch but the POI is 5 o'clock from POA, then your window is closed.
Your window will be then 1.844" down to 1.838" which is a .006" wide window. At this point you can seat every round to 1.844" or 1.843" and more than likely the point of impact will be the same.
Lets say your throat eroded .001", your window basically shifts upwards by the same amount to 1.845" down to 1.839" but considering your window is .006", you still don't have to bother with changing your CBTO of 1.844" or 1.843".
The only time you would even bother is if your throat eroded .005-.006". In which case, simply add .005" to your CBTO (new CBTO 1.849" or 1.848").
The wider the window, the less you have to mess with this. For example if you find that your window is larger than .009", don't even bother tryin to find out how big it is. It could be as wide as .020" but all you'd do is just waste time confirming it.
But again, how much time do you really want to spend messing with this?