MoviePropMaster made a thread about this exact topic:
http://www.imfdb.org/forum/showthread.php?t=92
As far as "Equilibrium" goes, that movie was filmed in a couple of different countries, including Canada, Italy, and Germany. The guns used by the Sweepers most likely change depending on which country was used for filming, because the local armorers provided what was available. This is why the Sweepers in the movie tend to use G36s and Walther MPLs (for scenes filmed in Germany), Beretta AR-70s (for scenes filmed in Italy), M16s (for re-shoots done in Canada), etc.
I'm guessing the AR-70 to M16 and back switch was because the scene was filmed originally in Italy, but then when additional shots were done in Canada, the Canadian armorer who was hired to handle weapons for the re-shoots didn't have any AR-70s in inventory, so he just brought an M16A2 onto the set instead.