RE: #6 SHOT
 

#6 certainly can kill a turkey at 40yds. They do start to drop after 30 yds though. They don't have the same energy as #5 at 40 yds but the high velocity loads still have plenty of energy to kill a bird. I shoot #5 because they don't really start to drop unitl after 40yds. If you have always used #5 and like the way they pattern then I would stick with them.

RE: #6 SHOT
 

Pellet count is more important than shot size (4,5 or6). My Benelli SBE with an Undertaker HS tube shoots Remington Hevi-shot #5 the best, even more pellet count than #6 shot. Find a load you gun likes and stick with it.