With premium match grade ammo a good quality iron sighted ad can shoot under minute of angle in the hands of a skilled shooter.
At camp perry national matches it is not unheard of for top shooters with iron sights to shoot groups under 5 inches at 600 yards.
At 100 yards an iron sight rifle ( again given a top quality rifle with premium ammo) sub 1 inch groups are possible. It takes lots of practice and trying different ammo( usually hand loads) to get there. I have personally witnessed a top shooter shoot a sub 2 inch group with irons at 300 yards