I have a mec 600 jr mark V loader. It has a 302-12 charge bar with a #12 powder bushing. I realize I need a 302- 1116 bar but I cant seem to find out what powder bushing I need to load 3in shells. Is the #12 bushing even the right one for 2 1/2 shells? I am using alliant green dot powder as per suggested by many other people. I guess over all I would like to load a high quality 3in load properly.

Go here and read everything you can on charge bars and bushings before you load anything.
http://www.mecreloaders.com/PowderBushingBar/BushingChargeBar.asp