Thats a great idea for a study if it has not been done yet I would be intrested in reading the results of such a study.
That said I had a 21 inch barrel with rifle length gas and it worked fine, my 18 inch barrel works fine with a rifle length gas as well.
I know JP uses a extra long gas tube with there 6.5 creedmoore barrels so maybe there is something to be had by exploring a 20 and longer inch barrel with longer then rifle length gas tubes
Edit.....Rico hit it on the head I think with the adjustable gas block, forgot to state that I also had ajustable gas on both my barrels.
That said I had a 21 inch barrel with rifle length gas and it worked fine, my 18 inch barrel works fine with a rifle length gas as well.
I know JP uses a extra long gas tube with there 6.5 creedmoore barrels so maybe there is something to be had by exploring a 20 and longer inch barrel with longer then rifle length gas tubes
Edit.....Rico hit it on the head I think with the adjustable gas block, forgot to state that I also had ajustable gas on both my barrels.