With good factory rifles it's easy to demonstrate grouping differences where significant runout is the variable. Of course a bad rifle won't shoot anything well. So you'd expect that a good rifle and straight ammo are needed to get a ten shot MOA group. I've read of crooked ammo shooting well, but that's the exceptional case.
I don't know if a custom rig can "force" crappy ammo to do better, but it seems one goal of a custom is to have it shoot everything better than a factory rifle.
