Different loads, style of loading and slug selection can have an enormous impact on regulation especially with smooth bores.
Sometimes I prefer less accurate slugs as they seem to overcome regulation by being more consistent. I'd rather a 6" group at 50m then two 2" groups 3" apart.
I view it more as a tighter pattern then as a group. I've also noticed that factory regulated guns aren't always better then playing with loads with a less regulated gun.
Once you find that butter zone many guns become great shooters.
But you have to find that load before you start to mess with the gun too much as some just won't behave and some turn out to be exceptional.
Many loads that are brilliant in a, single barrel gun just don't regulate in a double, but less accurate ones perform brilliant in a double.
But every now and then you find something that's just magic.