With so many different opinions on the M4A1 issue, I see a perfectly reasonable solution to the problem.
Funzio says that a minority of users have farmed the M4, and that their error in the stats was leading to a disadvantage against those that don't farm them.
My solution:
Bring back the M4A1 to it's original stats and create a mission called "Farm the M4A1". Give that mission to everyone that has passed the boardwalk unlock step, and to new players as they reach the boardwalk unlock stage. As part of the Mission completion message indicate that players can return to farm the M4 at any time to boost stats.
Boom. Everyone is happy.