Luke,
Consistency is the name of the game when reloading for accuracy, which I feel is the most important reason for taking up this endeavor.
The way my mind reasoned it out; different OAL's for one load (117gn Sierra/52.0gn H4831) will create different pressures inside the chamber area of your rifle. This in turn creates different velocities and "barrel times" for those 117gn pills, which envokes different barrel harmonics. However slight these harmonics differ will create different points of impact for your bullets. Thus accuracy suffers.
I feel similar results could be obtained by keeping the OAL the exact same but varying the powder charge a grain or two on either side of the optimum charge, or accuracy node for your rifle.
Other things affect consistency also, such as temperature, humidity, altitude and barometric pressure. You can't expect your loads to print the same in December (30deg) as they did in June (98deg). Moderen Smokeless Powders are good, but not that good, and I don't burn one brand. I have approximately 20 different "rifle" powders and (half that many "pistol" powders) all create different "barrel times" in my rifle(s). This allows me to find the best powder for whatever weight bullet I decide to load for. Example: I wouldn't use Retumbo (although you probably could) to load the 75gn Sierra HP in the .25-06, you may not be able to get enough in the case.
Different bullet weights equate to different lengths. Some can be set at .010" off the lands and grooves and others will have to jump more. Good luck. JohnnyK.
