"but is it common for factory ammo to shoot so poorly sometimes?"

Each gun is an individual, and what one will shoot another will not. Obviously factory ammo is a compromise. They do their best (usually!!), but often that is not going to work in your rifle. No matter how good factory ammo becomes, it cannot be a custom fit to every rifle. That's why factory ammo will never replace good handloads when ultimate performance is the goal. You don't see the benchrest boys using factory-loaded ammo!

I have never gotten decent accuracy from Remington factory .243 ammo, or fromRem. .22 Hornet stuff either. But I have had some very fine accuracy from Rem. Corelokt BULLETS in some of my handloads.