Generally speaking the inside of the shell is larger than the nominal bore so the column will actually be a bit shorter prior to entering the bore proper. "IF" the thickness of the shot collar inside resulted in a diameter of the bore of about .715" then 1 oz would be square. I have quite successfully fired 1oz of shot from 12, 16 & 20 gauges, a favorite shot load for me over the years for my hunting. Obviously it is not square in all of them but it just never bothered me.
The fit & feel of the gun seemed to always have a lot more to do with my
hit-a-bility than did the gauge or whether the load was Square or Skewed.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra