Many movies filmed in the US use airsoft guns with edited in muzzle flash, report and shell casings when very modern FA's are needed or when there's a budget. It would be possible for real ones to be used, however the SOT would need a LE love letter for each post sample he has- and then, he would have a very limited numbr.
Many films that include real post 86 machine guns are filmed overseas- Canada is much more accomadating. There is pending legislation right now to help facilitate (among a bunch of other things) the posession of post sample by professional theatrical and film organizations.