I purchased a Ruger Precision Rifle in 556/223 about 18 months ago (i.e. before they discontinued it in that caliber). It has a 1:7 twist rate. Have been to the range with it six times and shot at 100 yards. My primary objective during all those visits was to test various brands and grain weights of ammo, most of which were match grade ammo(think $1-1.25 per round), to see what performed best. Then I could focus my purchases on just those. (Btw, I regard myself as a novice precision shooter. However I did stay in a Holiday Inn, lol.) Shot from a bench with a bi-pod in front and a mono-pod in rear of rifle.
I compiled the results into a spreadsheet, and fortunately a pattern did emerge. As expected 70 and higher grain weights performed best.
Not sure if these results will apply equally to a semi-auto AR with 1:7 twist. Perhaps barrel length may also matter.
The results are shown in the screenshot below. Those in Red are all over 1 MOA. Thus off my shopping list. Notice they were generally the lighter grain weights.
The two standouts are the SigSauer #E223M1 and Hornady #80267 which averaged 0.678 and 0.784 respectively. The Hornady is a bit less expensive at .75 per round. whereas the Sig is $1/round. For me, rows 2 thru 6 are now my "go to" rounds when shopping for precision ammo for my RPR.
There is still a place for the cheaper 55 grain stuff. E.g. for warming up, shooting steel plates at 100 yards, etc. So will usually also have some of that on had.