Sgt. Joe Enders (Nicolas Cage) uses an M1928 Thompson fitted with a 50 round drum during the opening firefight in the swamps of Guadalcanal.
M1928A1 Thompson
For the remainder of the film, Sgt. Joe Enders (Nicolas Cage) carries an M1928A1 Thompson fitted with a 30 round magazine. Sgt. Pete "Ox" Anderson (Christian Slater) also uses an M1928A1.
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).
Springfield M1903
Some of the Marines during the opening scene use Springfield M1903 rifles. USMC indeed fought with them in Guadalcanal, while US Army was issued with M1 Garand. It should also be noted that M1903 rifles were commonly used alongside M1 Carbines and Garands due to M1 Garand shortages and that a number of front line Marine units were still using M1903s as infantry rifles up until the end of the Second World War due to the shortage of M1 Garands.
Machine Guns
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.
Browning M1919A4
Browning M1919A4 machine guns are seen both pintle mounted on US jeeps, mounted on M2 tripods in the hands of the infantry as well as in a coaxial mount on M4 Sherman tanks.
Browning M2HB
Browning M2HB machine guns are seen mounted on US M4 Sherman tanks.
Handguns
M1911A1
Both Sgt. Joe Enders (Nicolas Cage) and Sgt. Pete "Ox" Anderson (Christian Slater) use M1911A1 pistols as their sidearms. One of the mistake is that the M1911A1 usually have 4 magazine as extra ammo, not two, like in the movie.
Shotguns
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.
Other
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.
M2-2 Flamethrower
Private Harrigan (Brian Van Holt) uses an M2-2 Flamethrower as his weapon in the film. True to Hollywood Pvt. Harrigan's M2-2 Flamethrower explodes due to a single shot from an FMJ, engulfing the Private and forcing Sgt. Joe Enders (Nicolas Cage) puts him out of his misery with his .45. In reality the fuel inside of a flamethrower is often difficult to ignite, which is why built-in magnesium igniters are used. The gas used to pressurize the tanks is also non flammable. In real life a hit from and FMJ or lead round would most likely knock him off of his feet due the massive pressure released from the small entrance hole. Although it should be noted that had incendiary ammunition been used there is a chance of an ignition of the tank/operator.
M7 Smoke Grenade
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.
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 due to John Woo logic, he is no match for Sgt. Joe Enders (Nicolas Cage) and a quick hip fired burst from his Thompson.
Arisaka Type 99
Arisaka Type 99 rifles are the most commonly seen rifles in the hands of the Japanese Imperial Army. The early models contained a folding wire monopod, flip up sights with "Anti-Aircraft" bars, chrome plated bores, and a dust cover. As the war went on the quality had dropped. These late war rifles where known as "Last Ditch" rifles and were very crudely manufactured with most missing the accessories of the earlier models.
At one point in the film, Pvt. Ben Yahzee (Adam Beach) disguises himself as a Japanese soldier to acquire a radio after his is destroyed. He also takes a substitute standard ("last ditch") Arisaka Type 99 along with the disguise.
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. Some have 20 round magazines.
Other
Type 97 hand grenade
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 Mortar"s (technically grenade launchers) are used by Japanese soldiers during the unexpected assault on the Marines in the controlled town.