Superbad459

Which choke to fire slug should I be using ? Currently only have the choke that it came with, Modified Three notches.

I just brought home a Benelli M4, I Field strip and cleaned and lub it up last night And I can hear gas piston rocking back-and-forth as i lifted the barrel up and down. Is this normal? I followed the owners manual and I watched a YouTube video on how to do it as well.