Hit on something this past weekend, and confirmed it today, that I thought I would share with the AR-15 crowd here...
I have both 1/9 and 1/7 rifles. All Colts. My duty and HD round is the Federal 62 TBBC. I also carry the Sierra 69 SMK/TMK round for it's long range accuracy. For my SBR, Hornady TAP 75 SBR is the default load. I have other ammo, including plain old 55 FMJ for practice, but those are my "go to" rounds.
After shooting this past weekend, and in trying to resolve an issue with a Vortex 3X Spitfire, I learned by pure dumb luck that with my primary loads being at 62, 69, and 75 grains, and shooting through both 1/9 and 1/7 barrels, that I can simply use the 69 SMK as my "Universal Zero" round, and the POI out to 100 yards will not move appreciably with the 62 TBBC or TAP 75 SBR rounds. Trust me, I tried it out of 4 different barrels, two 1/7 and two 1/9.
FYI, standard 55 FMJ tends to fly a notch high and right for me out of all four, but if I dial in on any of my barrels with the 69 SMK, my POI stays within roughly 1 MOA from the 62 TBBC to the 75 SBR. Mind you, out to 100 yards. Beyond that, I don't know.
Maybe it's just me, or my rifles, or my barrels, but from now on I only feel the need to zero any rifle on that one round. If I'm on with that round, I'm on with any round out to 100 yards. Which works for me anyway, since the 69 SMK/TMK is my default round for any shot I know is in excess of 100 yards.
Nothing's cheap, or easy to find right now when it comes to 5.56 ammo. So every round has to serve a purpose. With four barrels to dial in, and everything from a CQB Aimpoint PRO to a Nikon-P223 involved, I believe I have found a single, albeit somewhat pricey "zero round", that will let me zero with confidence that the round of choice through that gun will hit where it is aimed.
Be interested to hear from those of you who know a lot more about it than I do.