It is not remarkable it is the standard acceptable accuracy of regulation. Only companies that cannot or will not regulate their rifles to this standard claim that 'minute of grapefruit' is what you should expect from a Double Rifle.

If each barrel shoots to 3 minutes of angle and the groups of the point of impact of the two barrels overlays the other than that is what you get. Good regulation and consistent ammunition.

Doubles can change regulation when a scope is added, a stock length is changed, velocity and pressure is changed, bullet weights are changed or the temperature is substantially different than when it was regulated. They can be very sensitive in regards to shooting to point of aim. Not all Doubles are effected by a single change but most are by multiple changes. They can be brought back into regulation by changeing the ammunition pressures and velocities with patience and time.