Anti-UN fighters use rifles that are a mix of various AK variants as their main weapon, first seen in "Jungle". While they have the characteristics of an AKS-74U (or an AKMSU), their barrels appear to be based from the AK-47.
AK-74
AK-74s are used by anti-UN fighters in "The Stars on the Earth" when Shin's family is assassinated.
AK-102
Anti-UN soldiers inside the Auerstädt use the AK-102 as their main assault rifle in "Birdman".
Colt M16A2
US Army soldiers with Colt M16A2s appear in Shin Kudo's flashback in "The Ocean, The Wind and...".
GIAT FAMAS G2
UN Marines are armed with GIAT FAMAS G2s as their main assault rifle.
IMI CTAR-21
Shin and Edgar are armed with IMI CTAR-21s as their main assault rifle, first seen in "The Stars on the Earth".
Sniper Rifles
Barrett M82A1
A Barrett M82A1 is used by an anti-UN fighter in "Jungle". Nora also uses it to shoot down the rock Birdman in the same episode, having a AN/PVS-4 night vision scope.
Machine Guns
Browning M2HB
A Browning M2HB is seen used by UN Army Abrams MBTs in a flashback when Shin Kudo narrates about how the UN Wars started in "The Ocean, The Wind and...".
Gryazev-Shipunov GSh-30-1
The Gryazev-Shipunov GSh-30-1 is first seen with anti-UN Air Force MiG-29 fighters as its main machine gun.
M61 Vulcan
M61 Vulcans are used by UN Air Force F-14A Kai fighters as its main machine gun. They are also inside Phalanxes, serving as machine guns for the UN Navy's Asuka II and Arleigh Burke class destroyers.
M197 Vulcan
In a flashback in in "The Ocean, The Wind and...", a giant widescreen in Japan shows a news report with a Bell AH-1 SuperCobra armed with a M197 Vulcan.
M240
The M240 machine gun is used by UN Army Abrams MBTs as secondary weapons in "The Ocean, The Wind and...". They consist of a M240C mounted coaxially next to the main gun and an M240D mounted at the Loader's hatch fitted with a gun shield. It's also seen in a flashback in the hands of a US Army soldier.
Rocket Launchers
Mk 153 Mod 0 SMAW
Several Mk 153 Mod 0 SMAWs are used by UN Marines to defend Mayan Island in "Blue and Desperate Battle".