1977 mini-14 and have never shot anything but milsurp through it. Sometimes my 100 yard groups are squirrel size, sometimes they are raccoon size, I think that is mostly on me and my shooting than it is the gun or the ammo although I have shot some old dirty Korean stuff that was bad no matter how I shot that day. For iron sights I can't ask for much more than that with that gun and my old eyeballs. I won't be putting a scope on it.
I have a 1:7 twist 186-series so it tends to like the heavier stuff. I've had great results with Armscor 62gr and PMC M855, keeping a 30rd magazine in the 9-ring or better at 75-100yds. I wasn't shooting 5 shot groups for accuracy that time, just looking for consistency and having fun, as well as looking for heat-stringing after adding my barrel stabilizer.
I doubt I'll try the light grain ammo, though she shoots 55gr just fine. I have noticed that even with the Mo-Rod and a flash hider, the 55gr rounds groups tend to open up a bit more regardless of brand. I just loaded up some 69gr SMK rounds with Varget so I'm interested in seeing how that weight works for me.
I cut my 195 series ranch to 16.25" and put a flash hider on. Then I needed a scope, then to get a bit better I went to those 62 FBI training hollow point penetration rounds. Then I got some Milsurp 62 grain green tips and they performed better than the 55 grain also. I'm staying with 62 grain bullets for this rifle.
Start testing what is easy to get and shoot 45 gr to 69 gr, common brands of 55 gr. Best thing to do if you don't reload, try to tune your rifle to the most common ammo, adjust gas block, add a strut, trigger job, bedding or shimming.