Here is my take on 20g guns.
First, time has taken it's toll for sure. Even an Ithaca 37 20g is taller through the barrel/magazine than a Remington 17 by almost 3/16s. The lines of each gun next to each other are obviously different. As well compare a 2 1/2 1920 Fox to a BSS!
Second, different bores handle shot differently(Duh!) A Ruger Red Label with a 635 bore will pattern very nicely with most quality 3" shells and has enough heft to make a nice duck gun. Especially with modern non-tox.
Remingtons and others that hover around 620s shoot the baby mags quite well. I've taken 2 longbeards with the 1 1/8 oz 20g with hard 6s. These can be poison on late season pheasants as well.
Older guns like my Remington 17s and old SxSs really seem to prefer 7/8s. I load #7s specifically for these guns and they are the bomb for 90% of the shooting I do, to include a pile of pheasants.
I don't think it's fair to say a 20 is a 20 is a 20. They've changed allot over the years and I suppose if you want a truely trim gun then you need to look to the 1930s and back. I have to admit though that the Beretta O/Us I've handled are pretty nice!
