Help Shotgun round size help

[UncleFester]

So in my parents house we have a shotgun readily accessible, just waiting for a day that it's just me and the gun.

There's 70mm rounds that are in the house, too, and I'm wondering if that size is enough if I was to aim through the mouth and go for the brainstem, or if that's not a viable option.

Besides that, so awesome to have found this site of people who understand!