On second thought to save on recoil and expenses.. You could buy a box or two of the 1300, load the gun with the first shell 1300, second shell 1200.. If the sporting Clay range will let you load 3 rounds in the gun..Call for the targets shoot the two targets that are thrown and see if the gun functions properly. Some ranges have rules against loading 3..
For a break-in load The 1300 will make a difference.
If you decide to go with just the 1200 ft cartridges, and pay for a hundred rounds of sporting clays.. Have a backup gun in the truck.
The 1300s might not work the gun either..
The gunsmith might have some good advice also???
I'm sure this reads like authentic frontier gibberish,
hopefully you can figure it out..