Unles you are using extremely light loads in the 20 ga it will require slower powders for a similar load than a 12ga. By "Similar" I mean neither equal nor proportionate. Conmsidering the normal Skeet/upland load as 7/8oz in 20ga & 1 1/8oz in 12ga & the "High Power" load 1oz in 20ga & 1¼oz in 12ga, in both cases the 20ga would require a slower powder to keep pressures in line with similar velocities. Much of the bad press given the smaller gauges in early days I feel stemmed from the fact that progressive powders (slow) were not then available & pressures were often high &/or erratic in the smaller gauges.
