In a big recoiling rifle, regulation will also be affected by the recoil movement of the muzzles. The projectile will have travelled some distance in the barrel with it moving under recoil, before exiting.

This is obvious will say a slow moving .45/70 single barrel. A slower load may shoot HIGHER than a faster recoiling load. Because the bullet is in the barrel longer and it has whipped up higher before exiting the muzzle.

A side by side will also have sideways movement.

So theoretically the muzzles will have moved up and to the right comparatively for a side by side right barrel. This is taken into account when positioning the two barrels during regulation.

Putting a scope on a rifle, ie weight. can affect regulation.

Putting a gas break or magnaporting to reduce recoil WILL affect regulation.

It would be good to know how it shoots since its defacement.