Avengers: Infinity War is the third installment in The Avengers film series. The 2018 sequel finds the Avengers attempting to prevent Thanos (Josh Brolin) from acquiring the six Infinity Stones that he desires to use to wipe out half of all life in the universe. The film marks the third feature in the Marvel Cinematic Universe to be directed by Anthony and Joe Russo. Infinity War is also part of "Phase Three" of the MCU and this particular phase would culminate with 2019's Avengers: Endgame and Spider-Man: Far From Home.
The following weapons were used in the film Avengers: Infinity War:
Sam Wilson/Falcon (Anthony Mackie) continues to use dual Steyr SPP submachine guns as his main weapons.
Steyr SPP - 9x19mm
Sam Wilson/Falcon (Anthony Mackie) continues to use dual Steyr SPP submachine guns.
Sam uses his weapons during the fight in Wakanda.
M249 SAW Paratrooper
Bucky Barnes (Sebastian Stan) is seen armed with what appears to be an M249 SAW Paratrooper fitted with a SureFire 100-round casket magazine and a Leupold Mark 4 HAMR sight. This M249 is identical to the one used by Bucky during the climax of Captain America: Civil War, though it cannot be the same as it was destroyed in that film, realistically the M249 has some severe issues when feeding from AR-style magazines, namely that the mag release is finicky and magazines may fall right out. Perhaps Bucky made some functional changes to it, such as upgrading the mag release.
M249 SAW Paratrooper - a real blank adapted movie gun with bipod folded and a picatinny rail on the top of the feed tray cover - 5.56x45mm NATO
In a deleted scene, Bucky Barnes (Sebastian Stan) chambers what appears to be an M249 SAW Paratrooper fitted with a SureFire 100-round casket magazine.
Bucky fires his M249 SAW while aiming down the HAMR scope. Note that he's using the scope with both eyes open, a technique that takes some skill to master, but gives an advantage in engaging an enemy (except when using high power scopes for faraway targets), presumably he mastered this during his training to become the "Winter Soldier". It's a nice detail to sneak in. That or the actor has no clue how to shoot and he didn't think to close his left eye.
In a deleted scene, Bucky is seen on the left as Steve Rogers (Chris Evans) leads a charge in Wakanda.
Bucky armed with his M249 SAW while fighting with Rocket in Wakanda. This angle shows an excellent view of the HAMR scope and the shortened barrel of the machine gun.
Bucky holds his M249 SAW while fighting with Rocket (the Guardian's resident gun guy). After demonstrating the weapon's power, Rocket expresses his jealousy.
Bucky charges with his M249 SAW and yells to perform presumably whatever the Russian equivalent of a Banzai charge is. This angle shows a great view of the SureFire magazine.