Building anything for fun is always a reason unto itself. If wondering about usefulness, or perhaps better said, employing the full capabilities of a gun, there is that undeniable relationship between weight and bore size. In terms of hunting usefulness my cut off is 10 bore. I use one, and have never ceased to be amazed at it's lethality.
My max load is 140 gr FFg. With wheel weight cast balls, I have shot through a 400 # black bear , shoulder to shoulder and out again, dropping it on the spot. Honestly, for most North American hunting a 12 bore would probably do just as well. The 10 just throws shot a little better IMO if hunting geese. As far as handling extreme recoil is concerned, we all have an upper limit , and I know mine from seeing the cumulative effects it had on my 1000 yard BP match results. A P.A.S.T. shield solved that problem, but I have read historical studies which concurred with Dan P. There is a level of recoil to which no one is immune. That's why my 4 bore sits on a swivel !