First thing I learned when I started reloading was to totally ignore the book OAL's. They are just some arbitrary OAL determined to fit in most magazines and feed in most firearms. If you are developing a load from starting values you can increase or decrease the OAL quite a bit with no pressure issues at all.
If there is a cannalure I usually use that and develop a load from there. The manufacturer obviously intended the bullet to be seated to that point. If there isn't a cannalure I use a different approach. The rule I follow is to have at least one bullet diameter of shank (full diameter section, ignore boat tail sections) inside the neck (or fill the neck if it's less such as in a 300WM). Then make the round as much shorter as necessary to fit in the magazine and facilitate feeding. So far I haven't run into a rifle that doesn't work with this method. A longer OAL reduces the jump the bullet has to make from the case to the throat/barrel and usually helps accuracy.
I use a different method for reloading for benchrest accuracy where the ammo can be a lot more fragile (less shank in the neck) since it wont be handled roughly. It's often so long it wont work in a magazine and needs to be single-fed.
