M1 Garand rifles are the most common weapons in the hands of U.S. Marines. Most notably seen used by Private Pappas (Mark Ruffalo) and Private Nellie (Martin Henderson).
M1928 Thompson
Sgt. Joe Enders (Nicolas Cage) uses an M1928 Thompson fitted with a 50 round drum during the opening scene in the swamps of Guadalcanal.
M1928A1 Thompson
For the remainder of the film, Sgt. Joe Enders (Nicolas Cage) uses an M1928A1 Thompson fitted with a 30 round magazine (although the capacity is continually ignored, being a John Woo film). Sgt. Pete "Ox" Anderson (Christian Slater) also uses an M1928A1. Since this is a Woo film, whenever Enders or Ox comes running in, all it takes is a good hip fired burst from their Thompsons and all the Japs are dead.
Both Private Ben Yahzee (Adam Beach) and Private Charlie Whitehorse (Roger Willie) use M1 Carbines throughout the film. This is strange because usually M1 Carbines were issued to soldiers of higher rank, not first time translator privates. It would be more likely for them to be issued M1 Garands.
Actually the M1 carbine was devised for troops who did rear enchelon duties, it was meant to replace the Colt 1911 pistol as a type of "sidearm" meaning in the sense a firearm to be used for personal protection. But not for deliberate attacks. So it would make sense on why a translater who's main duty isn't fighting, to be given an m1 carbine for self defense when a pistol just won't cut it.
Browning Automatic Rifle M1918A2
Private Chick (Noah Emmerich) uses a Browning Automatic Rifle M1918A2 as his role of the support gunner in the film, although his BAR is firing on the faster auto fire rate and has the bipod removed, suggesting it is used more in the assault rifle role.
M2-2 Flamethrower
Private Harrigan (Brian Van Holt) uses an M2-2 Flamethrower as his weapon in the film. Using a flamethrower in any combat situation is a brave move as a single hit from an FMJ round to the tank containing a mixture of Napalm and Gasoline, and the tank will explode and engulf the user in flames. Harrigan is unlucky enough to experience this himself, forcing Sgt. Joe Enders (Nicolas Cage) puts him out of his misery with his .45.
Winchester 1897
Gunnery Sergeant Hjelmstad (Peter Stormare) uses a Winchester 1897 "Trench" gun as his weapon in the film, although he is never seen firing it.
EDIT: In the "Director's Cut", Hjelmstad uses it during the surprise Japanese attack on the village where Harrigan, Ox, and Whitehorse are killed.
Springfield M1903
Some of the Marines during the opening scene in Guadalcanal use Springfield M1903A3 rifles. While the M1903A4 was usually issued as a sniper rifle, sometimes '03 rifles were used due to M1 Garand shortages.
M1 Bazooka
An M1 Bazooka is used by Marines during the battle of Saipan. When two Marines are attempting to take out a bunker with the device, the loader is wounded and the gunner is killed, so Sgt. Pete "Ox" Anderson (Christian Slater) runs out and does a stylish "judo-roll", grabbing the launcher in the process. The wounded loader gets back up and readies the weapon, and Ox takes out the bunker single handedly. Some other Marines are also seen carrying them.
Browning M1919A4
Browning M1919A4 machine guns are seen both pintle mounted on US jeeps and mounted on M2 tripods in the hands of the infantry.
Browning M2HB
Browning M2HB machine guns are seen mounted on US M4 Sherman tanks.
Marines throw unknown smoke grenades to cover the approach of Sgt. Joe Enders (Nicolas Cage) when he charges a machine gun emplacement to throw a satchel charge inside.
Arisaka Type 38
Arisaka Type 38 rifles are used by the Japanese Imperial Army troops. At one point in the film, Pvt. Ben Yahzee (Adam Beach) disguises himself as a Japanese soldier to aquire a radio after his is destroyed. He also takes a Arisaka Type 38 along with the disguise.The rifles with the top wood cover are arisaka type 99 rifles while the ones without are either type 38 rifles or "Last Ditch" type 99 rifles
Arisaka Type 99
Arisaka Type 99 rifles are the most commonly seen rifles in the hands of the Japanese Imperial Army.
Arisaka Sniper Rifle
While not closely seen, a Japanese sniper takes out a few Marines with an Arisaka Rifle, assumed to be a scoped model to fit the sniper role. But do to John Woo logic, he is no match for Sgt. Joe Enders (Nicolas Cage) and a quick hip fired burst from his Thompson.
Nambu Type 99 Light Machine Gun
Japanese soldiers are seen mowing down Marines with Nambu Type 99 Light Machine Guns. When trying to steal a Japanese radio, Sgt. Joe Enders (Nicolas Cage) uses two Type 99 LMGs to cover Yahzee as he calls in.
Japanese soldiers during the opening scene in Guadalcanal use Type 97 hand grenades, one of which cripples Sgt. Joe Enders (Nicolas Cage) and damages his ear beyond use.
Type 89 "Knee Mortar"
Type 89 "Knee Mortars" (technically grenade launchers) are used by Japanese soldiers during the unexpected assault on the Marines in the controlled town.