In both films Bucky is depicted using just a regular M249 machine gun, and being decently effective in battle, particularly against Outriders.

My issue is that pretty much any military age male is capable of handling an M249 without much issue. This means that really any decently sized military could’ve been effective against Thanos’ army in both films, at least against the ground forces like the Outriders.

By the way, the M249 is chambered in 5.56mm, which is not a big caliber by any means. In hunting circles it’s barely rated for whitetail deer (this is the .223 which is the civilian version, but it’s pretty much the same).

Of course, the obvious answer is that this is a superhero movie.