The Expanse show features numerous weapons: ship-based, small-arms, and improvised.
Miller's revolver (Image 1)
Recovered from UN Black-ops tactical team, used to kill multiple hostiles, including Dresden, Filat Kothari, and multiple mercenaries on Eros. Based on the Chiappa Rhino 50DS (.357 magnum)
The 1st Gen "Martian Pistol" (images 2 and 3) Martian sidearm used by multiple characters, based on the WE X-series airsoft gun, which is in turn based on the Springfield XD-9 9mm.
Star Helix Pistol (images 4-5)
Sidearm used by multiple characters, based on the Glock 26
2nd Gen Martian Pistol (image 6)
Briefly seen in Season 2 episode 1 on MCRN Siriocco.
OPA SMG (image 7-8)
Seen in assault on Thoth Station, based on Kriss Vector (.45 ACP)
Star Helix Long Gun (image 9-10)
Heckler & Koch SL8-4 (5.56 NATO)
Umarex Walther Redhawk (Images 10 and 11)
Pulled by UN Tac-team operative.
Kriss Vector (Images 12 and 13)
SMG used by UN Tac-team operator. Extended magazine, removed buttstock.
APS UAR (Airsoft only) (Images 14-15)
Electronic rail-mounted sight. Helmet HUD integration?
PDC (Images 16-17)
Possibly based on GE M134 Minigun
Gel Gun (Images 18-19)
GL-06 40mm Grenade Launcher