Why a PM? I’m always on the lookout for new tips/tricks and appreciate when people share them. $.02
I was hoping to jump on the phone with Mick to describe the process as I think it'd be easier since this is all new to me.
This came from my gunsmith and I've seen versions of this same process on YouTube channels. Basically, if it's a Rem 700 or 700 clone (not sure of other actions), remove the firing pin and ejector and load a reloaded round that you know won't chamber due to the bullet being seated too far out. Then with your press and seating die slowly start seating the bullet deeper and deeper while each time loading into the chamber and gently trying to close the bolt. When the bolts starts to get close to closing start reducing the seating depth so it's only being seated 0.001" or less each time. This took me quite a few times, but once the bolt closed easily I stopped there and measure CBTO with a bullet comparator. This should get you a CBTO length of 0.001 or less off the lands (jam). My 280 ai had a CBTO measurement of 2.8435 and I loaded bullets to -0.050 for a CBTO of 2.7935 and a COAL of 3.4420..
I have the Hornady COAL and a modified case for my gun, but I was finding that I was getting a different measurement each time I push the rod/case/bullet into the chamber. What I think was happening is the modified case wasn't fully seating in the chamber each time due to how tight it was.
After this process, for my 280 ai I loaded some Hammer 143 gr bullets 0.050 off the lands and ended up with a cartridge that has a COAL 0.190 longer than a Nosler factory loaded round (in photo, 143 Hammer on left and Nosler eTip factory on right), but easily fit my magazine and with room to spare. I loaded up a five cartridges in each 0.5 gr increment from 59.0 to 62.0 grs and I'm heading to the range tomorrow to see which is grouping the best. I may then start playing around with seating depth, 0.030, 0.020, etc., to fine tune accuracy.
I'm brand new to this myself and I'm working through the process and absorbing as much as I can. Both CAhunter805 and PCPython have been helping me and answering my ridiculous questions. Plus my wife has to be tired of me watching YouTube videos on reloading by now!! Any added advice to this process is appreciated.