I have two 1:9 twist Savage .223's : a Stevens 200 and a 110 FP. Both have original factory barrels.
Both absolutely love bullets in the 62 to 69 grain range - they will shoot anything in that range like a laser. 1/4" groups at 100 yards....3/4" at 200. Probably do better at 200, if I had younger eyes.
55 grain have always been mediocre, though. Just can't seem to find a combination that produces really tight groups.
I haven't tried anything lighter than 55 grains, though, on the theory that they probably won't work. Can't say for sure, though. Well, to be honest, I have tried some 50-52 grain loads at the range, a few times. Those loads shoot well in my shooting buddie's Savage 12. But, they were pretty mediocre in my rifles.
Heck, I've even tried 75 grain in these two rifles. They stabilized (no keyholing).... but I think just barely. Best accuracy was 4 -5 " at 200 yards....so I dropped them pretty quickly.