After buying an AR10 clone (Remington R25) 4 years ago, I bought an RCBS Precision Mic in .308, and quickly found that I wanted the Hornady/Stoney Point tool as well, after finding the bullet gauge RCBS includes for measuring bullet ogive not all that useful.
Admittedly, I hadn't had ANY experience loading for military-style semi-autos before, and after reading a lot of references....including the very good section on gas guns in my Sierra manual, not to mention, an article from a well-known AR clone manufacturer, attempting to scare people out of reloading for AR's (typical), I realized that dealing with head space correctly with these guns was a pretty important key to keeping them (and me) in one piece using reloads.
Anyway, I quickly found, that for that rifle, OAL was a moot point since the limitation was magazine size, not chamber size....still, the Stoney point tool helped me find that out. The most important thing was setting the shoulder back enough, but not too far to overwork the brass.....and for that the RCBS tool was just the ticket.
A comment about Varmint Al's nifty article:
Primer setback should be an obvious result of primer ignition....the flash hole is tiny, like the nozzle on a rocket engine. All the thrust is behind it. Although the intent isn't to make the primer fly, it does.....as far as it can.
Reminds me of my first homemade rocket I built as an 8 year-old stupid kid. I didn't understand nozzles....I just packed in my 7-1 ratio of powdered sugar and potassium nitrate into a tube and lit it. The result was a 6 " lift and a big burned circle in my mom's prize blue grass lawn.