Whereas the powder charge information in manuals should be followed carefully, the COAL is more of a general guide and something that needs to be tailored more to suit your rifle. I assume the "standard" COAL is used more by commercial ammo makers to give them a standard reference and also give the gun makers a standard reference for building magazines and creating chamber/throat lengths.
I have personally found that the order of priority in finding a good load for a rifle is: bullet selection, then powder/charge as a close second, then COAL variance (i.e. specifically the ogive-rifling jump), then primer. Case brand selection is another factor, having more to do with brass uniformity. I find that as long as you do load development with a particular brand of brass, properly prepped/uniformed and sorted by weight, then you will get good results.
What is important in fine-tuning loads for your particular rifle is determining how much "jump" your rifle seems to like for bullets. That is, how much travel the bullet takes as it begins to move out of the neck of the case and jumps to engage the rifling (first contact to rifling with the ogive of the bullet). Most guns shoot their best groups with a minimal "jump". Often target shooter will load rounds so that when you chamber them, the ogive of the bullet is actually touching or, in some cases, even pushed slightly into the rifling. This is a bad idea for hunting rounds, as you run the risk of the bullet getting stuck in the rifling and being pulled from the cartridge when un-chambering loaded rounds (especially if the bullets are not seated far enough into the neck). Also, most magazine-fed rifles won't fit rounds loaded that long anyway (this does not affect the target shooter or single-shot owner who loads single rounds directly to the chamber). In cases of light grain-weight bullets (for their respective caliber) you may sometimes find that the bullets are just too short to seat out to engage the rifling, without having them barely in the neck of the case.
So, if you have done some experimenting and found a particular bullet/powder charge that your rifle likes, the next step is the time to tinker and find the particular COAL that shoots best. Load up ammo with the same bullet/powder/case/primer and try 5-round sets (I prefer 5 shot groups for statistical data, as they are just a little more reliable than 3 shot groups) with varying bullet ogive to lands jump. For a hunting rifle, try starting with a jump of about 0.01" and increase it in 0.005" increments (0.01", 0.015", 0.020", etc.) until you hit the best combo. There are a couple of very handy tricks/tools to use when determining the COAL required with a particular bullet, to have the ogive touching the rifling lands when the round is chambered (this will be different with each individual rifle). I'd be happy to help you with determining that, but it would get a little long winded and might be easier on the phone. Feel free to PM me if you want some help on this.
Another trick to save a little on all this reloading/testing can come from results you may already have with factory ammo. Record the bullet jump distance for each factory load. If your gun shoots like a house on fire with a particular load, use the ogive-rifling jump distance of that load as a starting point.
The important conclusion is that once you find an ogive-rifling jump that your rifle seems to like, with any bullet, you will likely find other bullets are at their best in this same range.
Here's the safety note... once you have a powder/bullet combo to try, and you are working with the COAL variable, start your shooting with the shortest COAL first (longest bullet "jump"). Watch carefully for signs of excess pressure as you shoot the increasing COAL cartridges as pressure can increase as the bullet jump decreases, since the bullet has less and less momentum to engage into the rifling. Never take a max or near max loaded round (by manual guidelines) and seat the bullet at or into the rifling, as the pressure spike needed to get the bullet moving with the resistance of the rifling can be dangerous. If you want to load rounds right at, or into, the rifling, make sure to back the powder charge down 10-15% and build it up again slowly with that particular COAL.
COAL and determining the best ogive-rifling jump for a particular rifle is not all that complicated, but it takes a lot to write about all the little details. Good record keeping is your friend, and always remember that every rifle (even if the same make, model, caliber) is different and data you generate is very specific to that particular gun. So, if you have a lot of guns come and go, keep separate log books for each and record the rifle's serial number on the log. If you get the rifle rebarrelled, you're back "square one" on determining that guns preferences.
I hope this info wasn't too long winded or confusing? Happy shooting!
