here we go again... yet ANOTHER M4A1 thread~ but hey, it's one powerful weapon and it's free! so, i don't blame you for starting a thread.
anyways, let me help you by providing empirical data. i conducted an experiment a couple of weeks ago looking at drop rate at boardwalk (also tried junkyard but quickly realized that the drop rate was really, really terrible). so let me share with you my finding:
since nov 19 (18 days observation), i have collected 97 M4A1s. that's 5.4 per day. but this is clearly not the best measurement because there are times i forget to login to play and of course i have to sleep (my energy bar fills every 5 hours). so i have used 3 other metrics to evaluate drop rate- mission, experience and energy.
1. mission: i've done 656 missions to get 97, equals 1 drop per 6.8 missions.
2. experience: 6.8 * 43 = 292, which means you will have gained 292 experience by the time you get 1 M4A1.
3. energy: 6.8 * 135 = 913. here the interpretation is you will expend 913 energy to obtain a single M4A1.
all three can be applied when comparing drop rates between different players with different playing styles... the same cannot be said of drop per day.
also, for comparison between players hitting the same area (e.g. boardwalk), metric #1 (mission) is sufficient. but, if you want to make comparison between boardwalk and junkyard to find which has the most efficient drop rate, i would go with #2 and #3.
hope this helps!