Mac had excellent comments on Double regulation from a previus post--
Take a barrel set from a double rifle, and lock them in a padded vise, with the sights on target at the distance it was regulated for. Then place two empty cartridge cases , without primers, in the chambers, then look through the primer holes, like a PEEP-SITE. What you will see is, the RIGHT barrel will be looking at a point to the LEFT of the POA, and LOW , and vise-versa for the left barrel! This is necessary because when the RIGHT barrel is fired, the rifle will rise up, and to the right, and again the left barrel will do the opposite, and the amount of time the bullet stays in the barrel durring recoil, will dirrectly effect the POA on the target, at a given distance! If the load is too slow, the group will be high & wide, if too fast it will be low, and crossed. Change anything in the load, or the weight, and ballance of the double rifle, and you will likely open, or close, and the composite group be high or low!

Ant pearls on regulating a SCOPED Double with a lighter faster bullet?? I am thinking of trying my 500 NE with a 2x red dot at 75-100 yds.I understand the weight of the scope may affect the regulation by more weight and ??lower the bullet paths due to decreased time in the barrels.

Any pearls besides loading up some 540gr GS solids and blasting away?