The 1st Gen "Martian Pistol", heavily modified version of the WE X Series IPSC Airsoft Pistol, itself based on the Springfield Armory XD continues to be used by James Holden (Steven Strait), Amos Burton (Wes Chatham) and Alex Kamal (Cas Anvar) after they acquired them in Season 1.
Note: In the context of the show, the weapons are stated to be "designed to fire both self-propelled rounds and traditional cartridge rounds, the rear of the barrel is vented at the back to eliminate recoil for use in zero-g situations".
Glock 26 Advanced
The "Star Helix Pistol", which is actually a visually modified Glock 26 Advanced Airsoft pistol (a fictional variant of the Glock 26 with an accessory rail, grip extension and compensated extended barrel originally manufactured by Tokyo Marui) continues to serve as the sidearm of many individuals in the series. This includes Cyn (Brent Sexton), Karal (Olunike Adeliyi), Camina Drummer (Cara Gee), members of the OPA Navy and Free Navy, and Chesapeake Maximum Security Correctional Center guards Rona (Natalie Brown), Morris (Arnold Pinnock) and Sullivan (Thomas Mitchell).
Note: In the context of the show, they are described as a "common gun made from plastic polymers so it can be 3D printed cheaply and reliably. It is designed for reliability and ease of use".
Tippmann TIPX (Olive)
A "Heavy Belter Pistol", which is in reality a modified Tippmann TIPX paintball pistol, is carried by several Belters and members of the Free Navy throughout the series. This includes Marco Inaros (Keon Alexander) and Filip Inaros (Jasai Chase-Owens).
Glock 17
An unmodified Glock 17 pistol is pulled by the head of Winnipesaukee Island Security (Paul Fauteux) in "Winnipesaukee" (S5E09).
The FN P90 is carried by Baltimore crime boss Erich (Jacob Mundell) and several members of his gang throughout the series.
Rifles
Heckler & Koch SL8-4
The "Star Helix Long Gun", which is actually an Airsoft Umarex SL9 resembling a Heckler & Koch SL8-4 with a G36K length barrel, handguard, and various futuristic modifications serves as the rifle of choice amongst many factions in the series. This includes the United Nations Navy and gang members working for Baltimore crime boss Erich (Jacob Mundell).
Assault Rifles
APS UAR
The "Martian Long Gun", which is actually a slightly modified APS UAR Airsoft rifle, is carried by several Free Navy members loyal to Marco Inaros (Keon Alexander) throughout the series. It is implied they have access to the weapons due to several Free Navy ships being supplied by rogue elements of the Mars Congressional Republic Navy.
IMI Tavor TAR-21
An unmodified IMI Tavor TAR-21 is carried by both a Winnipesaukee Island Security officer and a member of Erich (Jacob Mundell)'s gang in "Winnipesaukee" (S5E09).
Amos Burton (Wes Chatham) uses a visually modified Kel-Tec KSG shotgun in "Winnipesaukee" (S5E09). It replaces the UTS-15 he carried earlier in the previous episode, seemingly in a continuity error.
Other
Black Market Weapons Collection
Bobbie Draper (Frankie Adams) develops a large collection of illegal weapons, kept in a secure storage unit. The collection is first seen in "Exodus" (S5E01) and consists of both UNN and MCRN weapons that she has taken off the black market whilst investigating rogue Martian military operatives.