"Modern dimensions" usually denotes something like 1.5 DAC x 2.5 DAH. Witness the new RBL's advertised at 1.5 x 2 3/8. They're supposed to fit Joe Average.

Pre-WWII it was more common to see 1 5/8-1 3/4 DAC x 2 3/4 -3 DAH for off-the-rack guns intended for Joe Average. But it's not uncommon to see target guns and live pigeon guns of the day with higher dims.