My guess would be that the shotgun barrels are chambered for 65mm (2-1/2 inch) shells. You need to have the chambers measured to determine their length. Don't even consider opening the chambers to 2-3/4". Although you probably won't weaken the barrel significantly, why take the chance?
2-1/2 inch shells are readily available as loaded ammo or components. I use Fiocchi 16 gauge cases to load for my drillings using a slightly modified MEC 600 Jr.