The COL in a manual is NOT a recommendation. The most it means is that was the COL that testing was done at and, if you use the same bullet, pressures might be higher if seat deeper. Consider it no more than the minimum COL for that load data.
COL is mostly determined by the gun--what it feeds or what fits in the cylinder. Determine the working COL prior to loading just like you are.
The following is my boiler-plate response to COL questions:
Per Ramshot:
"SPECIAL NOTE ON CARTRIDGE OVERALL LENGTH “COL”
It is important to note that the SAAMI “COL” values are for the firearms and ammunition manufacturers industry and must be seen as a guideline only.
The individual reloader is free to adjust this dimension to suit their particular firearm-component-weapon combination.
This parameter is determined by various dimensions such as
1) magazine length (space),
2) freebore-lead dimensions of the barrel,
3) ogive or profile of the projectile and
4) position of cannelure or crimp groove.
• Always begin loading at the minimum "Start Load".
• Increase in 2% increments towards the Maximum Load.
• Watch for signs of excessive pressure.
• Never exceed the Maximum Load."
Your COL (OAL) is determined by your barrel (chamber and throat dimensions) and your gun (feed ramp) and your magazine (COL that fits magazine and when the magazine lips release the round for feeding) and the PARTICULAR bullet you are using. What worked in a pressure barrel or the lab's gun or in my gun has very little to do with what will work best in your gun.
Take the barrel out of the gun. Create two inert dummy rounds (no powder or primer) at max COL and remove enough case mouth flare for rounds to chamber (you can achieve this by using a sized case—expand-and-flare it, and remove the flare just until the case "plunks" in the barrel).
Drop the inert rounds in and decrease the COL until they chamber completely. This will be your "max" effective COL. I prefer to have the case head flush with the barrel hood. After this, place the inert rounds in the magazine and be sure they fit the magazine and feed and chamber.
You can also do this for any chambering problems you have. Remove the barrel and drop rounds in until you find one that won't chamber. Take that round and "paint" the bullet and case black with Magic Marker or other marker. Drop round in barrel (or gage) and rotate it back-and-forth.
Remove and inspect the round:
1) scratches on bullet--COL is too long
2) scratches on edge of the case mouth--insufficient crimp
3) scratches just below the case mouth--too much crimp, you're crushing the case
4) scratches on case at base of bullet--bullet seated crooked due to insufficient case expansion (not case mouth flare) or improper seating stem fit
5) scratches on case just above extractor groove--case bulge not removed during sizing. May need a bulge buster.
