The conventional wisdom is that you don't need to worry much about shell length or velocity or payload or burn rate or any of that other stuff as long as the pressure is appropriate for the gun. Most early Parkers and a lot of other older American doubles had short chambers (2-5/8") and are shot regularly with 2-3/4" shells. Lots of those shells are low pressure, in deference to the ages of the guns, but piles of standard target and promo loads get burned up in old guns. What you can/should use depends on the specific gun.
