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. Alex is briefly disarmed of his pistol by MCRN Ensign Loftis (Kelly McCormack) in "Reload" (S3E04).
A 2nd Gen "Martian Pistol", also a modified version of the WE X Series IPSC Airsoft Pistol with a slightly different design, is used by Dr. Praxideke Meng (Terry Chen) in "Triple Point" (S3E05). Variants of this were briefly seen aboard the MCRN Sirocco in Season 2.
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. It is carried by United Nations Navy personnel, including Fleet Admiral Augusto Nguyen (Byron Mann), Lieutenant Shaffer (Natalie Lisinska), Lieutenant Mancuso (Morgan Kelly) and Lieutenant Boyer (Shawn Ahmed). Belters such as Camina Drummer (Cara Gee) and Klaes Ashford (David Strathairn) also carry the weapon, as well as United Nations Military Police and briefly also by UN Deputy Undersecretary Chrisjen Avasarala (Shohreh Aghdashloo).
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".
Umarex Walther Red Hawk
The Umarex Walther Red Hawk is carried by several Protogen employees, including Dr. Lawrence Strickland (Ted Atherton), in "Immolation" (S3E06).
Submachine Guns
TDI Vector
A TDI Vector is carried by Egan (Obssa Ahmed), Amos Burton (Wes Chatham) and several members of the OPA Navy in "Abbadons Gate" (S3E13). Several other TDI's are also seen in the armory of the OPAS Behemoth.
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 for United Nations factions in the series.
APS UAR
The "Martian Long Gun", which is actually a slightly modified APS UAR Airsoft rifle, is carried Martian forces as well as the crew of the Rocinante throughout the season. One is briefly obtained by MCRN Ensign Ranhofer (Alain Chanoine) in "Reload" (S3E04). Gunnery Sergeant Bobbie Draper (Frankie Adams), Private Riko Oshi (Sabryn Rock) and Private Jed Trepp (Adolyn H. Dar) also use them in "Abaddons Gate" (S3E13).
FN SCAR-L
A heavily modified SCAR again appears as the auto-shotgun carried by Amos Burton (Wes Chatham). It is also carried by Dr. Praxideke Meng (Terry Chen) in "Triple Point" (S3E05). A magwell cover prevents the type from being identified.
Machine Guns
GE M134 Minigun
The Rocinante is equipped with six "Point Defense Cannons" that resemble GE M134 Minigun's.
Note: in the context of the show, the PDC's are said to be manufactured by Nariman Dynamics and chambered in 40mm.
Grenades
NTW-08 Hi Explosive Grenade
Several members of the Martian Marine Corps, including Lieutenant Paolo Mayer (Simu Liu) and Gunnery Sergeant Bobbie Draper (Frankie Adams) arm themselves with fictional MCRN NTW-08 Hi Explosive Grenade's in "Dandelion Sky" (S3E10).
Other
Goliath Powered Armor Minigun
A built in minigun is a key feature of the Martian Marine Corps Goliath MkIII Powered Armor. The weapon is seen used by Gunnery Sergeant Bobbie Draper (Frankie Adams) throughout the season, as well as Martian Marines Lieutenant Paolo Meyer (Simu Liu), Private Riko Oshi (Sabryn Rock) and Private Jed Trepp (Hamed Dar) in "Dandelion Sky" (S3E10).