Keeping in mind that the incremental difference anticipated as an improvement in accuracy could be achieved by a lot of other finesse, too. And also considering that the AR15 is a self loading action, which pushes the cartridge off the magazine into the feed ramp up into the chamber. A loose neck or loading them without checking for coaxial runout could make it moot. OAL is an easy thing to exploit reloading, spinning each round to see how much wobble the bullet tip has is much less fun - but considered by some to be much more important. They spin and true the bullet nose to .001" runout.
OAL is just one of a dozen details in a reload which could affect accuracy, and if we are limited to just 12, calculating the total number of possible combinations is 12 factorial, 12x11x10 etc etc down to x1, which is 479,001,600 different loads to experiment which is the most accurate. And you shoot 10 rounds each to get an accurate idea of what it's grouping.
No wonder I can't find primers in stock.
