The M1903A3 Springfield rifle is the main weapon of the 1st Division Marines who land on Guadalcanal in part 1 and part 2, until they replace it with mostly Garands by part 3.
M1 Garand
Several Characters are seen using M1 Garand rifles, Eugene Sledge (Joseph Mazzello) pulls an M1 Garand from a dead body to shoot a Japanese soldier with a Katana. Most of the Marines have their 1903s replaced with these by part 3. One Marine had compared it to his 1903 and prefers than in terms of accuracy.
The Arisaka Type 99 rifle was one of the main battle rifles used by the Japanese Army in The Pacific theatre.
Submachine Guns
M1928 Thompson
Marines in Part 1 are seen with the M1928A1 Thompson. Lt. Hugh Corrigan (Henry Nixon) had a 50 round drum on his for most of Part 1 until the end, replacing it with a 30 round magazine.
M1A1 Thompson
Several Marines are seen firing an M1A1 Thompson. Many others are seen with Thompsons too. Robert Leckie (James Badge Dale) uses a Thompson M1A1 quiet a bit in Part 4, specifically when he takes out a group of four Japanese patrol that we're trying to flank his unit.
Reising M50
John Basilone (Jon Seda) is seen with a Reising M50 Submachine Gun in "Part 2".
Handguns
Colt M1911A1
The main sidearm of the Marines in the miniseries is the Colt M1911A1. Robert Leckie (James Badge Dale) and John Basilone (Jon Seda) are seen using the pistols and several troops have some holstered.
Nambu Type 14
The main sidearm of the Japanese was the Nambu Type 14 Pistol. Robert Leckie (James Badge Dale) finds one in a Japanese officer's chest when they go through the camp in which he takes it and the chest as a war trophy. After being committed due to the effects of combat, He ends up giving the pistol to the head of a naval psychiatric hospital as a bribe so he can return to his unit in Part 4.
Machine Guns
Browning M2HB
The Browning M2HB machine gun is used by the US Marines, mounted on LVT's.
Browning M1917
The Marines of the 1st Division that land on Guadalcanal use Browning M1917 water cooled machine guns. Robert Leckie (James Badge Dale) is seen firing it as part of a three man gun crew in "Part 1". John Basilone (Jon Seda) is also seen firing it both on a tripod and from the hip in "Part 2".
Browning M1919
Browning M1919 machine guns are seen mounted on LVT's in the series.