Basically, what I do is create a private multiplayer match by myself. You have to go into match options and select enable recording, and custom classes. From here you can create the custom classes, and when making a private match, all weapons/perks/etc. are unlocked, so you can make your classes whatever you want to rip. Then, once you have your classes all set up, you start the match. While in the match, use the weapon and go through all the motions you want to rip (ie. Select, Deselect, Fire, Reload, etc.) Then quit the match.
Back out to the multiplayer menu, and enter theatre mode. Select the match you just played from My Recent Matches, and select it for playback. Once you have it up, record the selections you want at a slow speed (I use 0.1x), and save your selections in your file share.
Now, back out and go to your file share and select your new film for playback. Now play it frame by frame (tapping A) and rip each screenshot (holding B). You can only save 6 at a time, so it's best to do this process with your laptop close by.
Now, you go to callofduty.com (registering is really easy) and go to the theater tab.

Your screenshots will be up so select the download button, and it will bring up a larger version of the screenshot.

Save the picture so you can edit it.


Then just rip the background using your photo editing program.

Once you get to this point just save it as a png file, and you can use it. If you are helping with my Doom Ops project, just put the screenshots in a WAD like this with the backgrounds ripped but without resizing them or moving them. I will fix it up from there. The pics should all be 2560x1440. Thanks!