I put together some summaries of the sales numbers for the last 6 Call of Duty games from VGChartz, and here are the results.
Unfortunately the VGChartz numbers don't include digital sales which means PC sales are lost and once a game goes up for sale on Xbox Live and the Playstation Store the numbers start to sour.
This shows the first 10 weeks of sales of each CoD game on PS3 and 360. You can see the first week makes the biggest difference between them. So let's take that out and see what the rest looks like.
BO2 was released later in the month compared to the rest of them so you can see it had it's Christmas peak a week earlier.
Another thing I see is how much convergence there is among all of the titles at the 9th and 10th weeks, particularly the steep decline of MW3 and BO2, which had Christmas week sales higher than CoD4's 1st week sales, but MW3 had a lower 10th week and BO2 had a lower 9th and 10th week than CoD4's 10th week.
According to VGChartz BO2's 10 week sales are lower than MW3 by almost 3 million, but higher than Black Ops by about 600,000.
Here's another one comparing the sales of each game in it's first year (Nov and Dec of its release year), second year (1st full year) and third year:
Added April 7, 2014: Charts updated to include Ghosts.