I have a good quantity of M193 and AE ammo, and I was intending to zero in the rifle with the M193.  

I've been browsing the discussions on which ammo is better for defense than the M193.  It's all clear as mud to me, MHP, OTM, BTHP, whatever.

From what I've read, since my rifle has a 1:9 twist 20" barrel, the 68 gr MHP Hornady bullet or the 69 gr of that style are the best rounds where I can expect good stabilization, correct?

Is the 75 gr round with a 20" barrel and 1:9 twist even worth trying, or is it like splitting hairs?

My other concern is if I zero in the rifle with the M193 ammo, how is that going to affect iron sight shooting with the 68 or 69 gr rounds?

What is the effective range of the 68/69 gr rounds with my rifle?

I don't really plan on having a case of the more expensive ammo sitting around but I'd like to have a couple mags worth.

A search would turn up a pile of threads with the info you so desire, plus by searching you'll realize one other thing: in your post, you already answered a couple of your own questions!