according to most of the research I have read, after 14" of barrel length the velocity return starts to "diminish". Thats not to say that the extra inch or two doesn't help at all.
I plan on trying some slower powders like 2400 or lil'gun to boost the performance a bit. With slightly larger powder charges of slower powders, you will get more gas production over a longer time frame, thus make better use of the extra inches of barrel for the additional acceleration over the extra distance.
My guess is that 20.5" is a little much, but I can cut it back any time I choose. Gonna be funnnnnn!
