Hi ENENEN
Since you got the Stag could you provide a little feedback?
I saw RRA has an 18" bull barrel that is right between the 16 and 20 you mentioned.
I also read a great article that summed up the velocity vs length dispute quite well:
For every 2" of barrel you gain or lose ~45fps of velocity (at the barrel), so your 16" will be ~90fps slower than the 20" (I know there are many other variables like twist and manufacturer, but all things being equal, with the same load, this is what you will get.)
Thanks