Rambo was originally going to fire that Browning M2 Aircraft from the hip in the fourth movie, but they found it was too slow to manuever and holding it was tiring for Sly so they slapped it on a truck.
And you can't really "shoulder fire" an M2, it has no stock, just two spade grips.
Personally I think they could depict a Terminator firing an M2 easily if they just used some wires (deleted in post-production of course) to make it look like they are flawlessly wielding them.
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