Kutter;
Is that Stevens 200 the model which had the short breech bolt & consequently a much shorter receiver than most pumps or semi autos giving it a shorter overall length for the same barrel length. If you can post it would love to see a picture of it.

I believe that at the time these guns were built the 3" 20 gauge shells still only carried an ounce of shot, just gave a better wad column when bulk powders were often used. I would think you're very wise not to stuff it with "magnums".

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