Okay i've looked through some of the threads pertaining to my question....and sort of found an answer......So one of my m4 aeg's is most likely firing aroung 460'ish, on .20. If i use .30 how far will it drop the fps to, im thinking about 100. What would be perfect is if i could get it down to about 400-415, so if .30 drop it 100 then .28 should drop it in that area....hopefully.
And which piston would you guys say is overall the best;
Modify ultra piston
Deep fire titanium
Promethus hard piston
Live for nothing, or die for something
They can have my rifle when they can pry it from my dead cold fingers