A Taurus PT99 modified with a barrel extension and other accessories to make it look post-apocalyptic is used by both the Mariner (Kevin Costner) and Deacon (Dennis Hopper) as well as various Smokers.
Webley Mk IV Pocket Model
One of the Smokers can been seen breaking open and loading a Webley Mk IV Pocket Model while aboard the Exxon Valdez.
A dressed-up Heckler & Koch MP5A2 is carried by several Smokers including Deacon's right hand man.
Rifles
Norinco Type-56
A few Smokers are armed with unmodified Norinco Type 56 rifles.
M16 with M203
A Smoker is seen armed with a M16 rifle with a M203 underslung.
M1903A3 Springfield
The M1903A3 Springfield rifle is seen in the hands of Smokers during the Atoll attack.
Shotguns
Sawn-off Double Barrel Shotgun
During the attack on the Atoll at the films beginnings, one of the Smokers fires a Sawn-off double barrel shotgun
Mossberg 500
A Mossberg 500 shotgun with a wooden forend and a pistol grip is used by Mariner (Kevin Costner). A humorous thing is the fact that during his assault on the Exxon Valdez, he is wearing a Bandoleer with 12 gauge shells, yet he discards the shotgun without ever using the shells in the bandoleer, which he discards as well.
Remington Model 870
One of the Smokers are seen with the Remington Model 870 (with the same decorations as on the Mariner's shotgun), when they attack the atoll.
Machine Guns
M60 machine gun
An M60 fitted with a barrel jacket (in the style of aircraft MG 08/15 "Spandau") is mounted on the seaplane.
Browning M2 Aircraft Quad Mount
A Browning M2 Aircraft Quad Mount is used by the Smokers on the siege of the Atoll, jamming once before resuming fire.
The Mariner then attaches a harpoon to the hull of the Smoker's ship, causing the Quad Mount to spin and take out the Deacon's boat.
Other
Type 90 Signal Pistol
A Type 90 Signal Pistol is used as a Shotgun Pistol by a Smoker during the raid on the Atoll.