I want to get a little advice from those of you who have a 5.56mm rifle, and, in the event of an emergency, may intend to use different types of ammo through the same rifle. I was thinking that if I standardized my ammo supply with two types of ammo (light, cheap and plentiful vs. heavy, pricey and lethal), I’d have to have two different zeros set up for the rifle. I haven’t come to any good conclusions yet how would be the best way to switch zeros as quickly as I can switch ammo. I typically just get my zero with my A.R.M.S.#40 and the A2 front sight, and then set the Aimpoint dot to be in line with the Irons’ sight picture. Should I be counting ‘clicks’, marking one side of my front sight adjuster tool (to count revolutions), or would it make it worthwhile to just get a BUIS with click-adjustable windage & elevation?

Pick what ever you think you are going to use (i.e. 75/77 grain), sight in the gun correctly.

Then shoot with XM193 (or similar) and "SS109" to see where these impact, record the data and insert in your memory, on each box/case of the XM193 and "SS109" and if you wish on the gun (in the sock on a pice of laminated paper etc.).  Would always check with NATO "SS109" just so you know - even if you do not have any nor wish to get any (side note - the "SS109" is still an interesting 10% complement to the 75/77 grain loading IMO).

If you have 500 to 1500 rounds of 75/77 grain ammo, that really is probably good enough for anything you will ever need in a SHTF world.  Assume a double load out plus a mag is 410 rounds (12 thirty round mags, one more 30 round mag in the gun and one extra 20 round mag ;-).

The couple of extra cases of XM193 or "SS109" is probably not needed except for plinking and in any case you will already know where it shoots.  If one or the other shoots the same windage as the 75/77 grain loading then would pick that brand / load for the "other ammo".  Can always figure a different range / aiming poing (i.e. top/bottom of Aimpoint dot) if you are lucky.

P.S. Best option is as always get two guns - one for each load ;-)