Discussion

After the Battle of Guadalcanal was won and the Marine Corps was preparing for another round of ever more expansive campaigns for 1943, its divisions received a new organization. The E-Series was issued to replace the D-Series organization in April 1943. As far as the Rifle Company was concerned, the basic company-level organization remained the same, but there were several more significant changes at the platoon-level. Although following operations on Guadalcanal, the E-Series organization was actually the implementation of pre-war changes that the Marines wanted to make but were unable to due to time and resource concerns when they updated their D-Series organization in mid-1942. The E-Series Rifle Platoon actually vaguely mirrored that of an experimental organization trialled by the Marines in 1934 and issued in 1938, by drew back in 1941. Additionally, new weapons were coming online, including the M1 Rifle and M1 Carbine that had not been common in infantry units prior.

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At this time the Rifle Company consisted of 1 Company Headquarters, 3 Rifle Platoons and 1 Weapons Platoon. The Infantry Battalion would also have enough Navy Hospital Corpsmen (medics) from its Medical Section to provide 6 per rifle company. The organization as discussed here was effective from 15 April 1943 to 12 January 1944 when it was replaced with the F-Series organization.

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The Company Headquarters consisted of 2 Officers and 26 Enlisted men. The company overall was led by the Company Commander who ideally ranked as Captain, but was sometimes a Lieutenant. A Lieutenant acted as the company's Executive Officer (Second-in-Charge) who handled much of the administrative, rear-echelon workload, while the company First Sergeant was the company's senior enlisted man. A Gunnery Sergeant was added to the Company HQ as well, responsible for ordnance and logistics. Otherwise there was a contingent of rear echelon support personnel, including clerks, cooks, mess personnel, field musicians, messengers, a cobbler signalman, light truck driver, carpenter, and barber. The 8 Other Duty Marines in the Company Headquarters were essentially Riflemen that could be used as replacements as well as mess personnel (the billet was renamed to supernumerary for mess duty in May 1945).

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The Company HQ's jeep was retained, with a 1/4-ton trailer being added. This was to help carry ammo for the newly strengthened Weapons Platoon (it was increased from 2 Mortar Squads and 2 LMG Squads to 3 of each). The HQ also kept 3 spare Bazookas on paper for self-defense and bunker busting, although these wouldn't be available to deployed units until the very end of 1943 (after Tarawa) or early 1944.

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The Weapons Platoon consisted of 1 Platoon HQ, 1 Mortar Section and 1 Light Machine Gun Section. The Platoon HQ was 4 men strong, with a Platoon Commander (Lieutenant), Platoon Sergeant (Platoon Sergeant, equivalent to a Staff Sergeant outside of the infantry) and 2 Messengers (Private). Meanwhile, each of the Sections was led by a Section Leader of the rank of Sergeant and armed with an M1 Rifle and further subdivided into 3 Squads each. The Mortar Squads each served a 60mm M2 mortar and consisted of 1 Squad Leader (Corporal), 1 Gunner (Private), 1 Assistant Gunner (Private) and 2 Ammo Bearers (Private). The Squad Leader was armed with a Rifle, while everyone else was armed with the M1 Carbine. The Light Machine Gun Squads each served an M1919A4 machine gun from a tripod and consisted of 1 Squad Leader (Corporal), 1 Gunner (Private), 1 Assistant Gunner (Private), and 3 Ammo Bearers. Like in the Mortar Squad, the Squad Leader was armed with the rifle while everyone else was armed with carbines. In practice, both the Mortar Section and Light Machine Gun Section were often broken up, with 1 of each squad being detailed out directly to a Rifle Platoon. This differed from Army doctrine of keeping them together, generally speaking. This was most likely due to the unique environment of the Pacific Theatre, where the confines of the jungle forced platoons to take a greater role in fire and maneuver than higher echelons (this also accounted for the Marines' more rapid development and refinement of small unit tactics while the Army's small units stayed the same practically speaking throughout the war). 

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The company's 3 Rifle Platoons consisted of 1 Platoon HQ and 3 Rifle Squads each. Gone were the days of the D-Series BAR/Automatic Rifle Squad, with that squad's BARs and the Platoon's spare BAR being redistributed to the Rifle Squads. The Marines prioritized fire over maneuver while the Army weighed both equally and were much more willing to increase the official allotment of BARs to Rifle Squads over the course of the war than the Army was. Much of this came down to the environment, as well as the necessity of projecting as much firepower forward as possible to survive during an amphibious assault against a contested beach. 

The Rifle Platoon overall was led by the Platoon Commander (Lieutenant) who was assisted by a Platoon Sergeant (ranked Platoon Sergeant, equivalent to a Staff Sergeant in other trades). The Platoon HQ also contained a Platoon Guide (essentially an assistant Platoon Sergeant, named after their traditional role in bearing the platoon's guide-on in formation), 3 Messengers (allowing the Platoon Commander to send one out per squad simultaneously) and 1 Other Duty Marine who could act as a replacement.

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The Rifle Squads saw a bit of a shake-up in 1943 with the Marines finally able to make the changes they had wanted to make before Guadalcanal but couldn't. These changes weren't informed so much by combat experience on Guadalcanal itself, but the 1944 F-Series changes would. The Squad Leader billet was promoted to Sergeant from Corporal. This is actually something that the Marines had tested in 1934 and wanted to do when the war began, but were forced to keep their Corporal Squad Leaders due to manpower reasons. They were also given an Assistant Squad Leader of the rank Corporal to help with controlling the newly increased 12-man squad (it had been 9 men under the D-Series). In theory the squad could be split into 2 equal teams with a total of 2 BAR Men, 2 Assistant BAR Men and 6 Riflemen. However, this was not done in practice. Command and control was still flawed at this point for the Rifle Squad, with the Squad Leader being both responsible for directing his Assistant Squad Leader and the rest of his men. With no solid fire team doctrine at this point, this actually increased the workload on Squad Leaders rather than decreasing it, which would be cause for change in 1944.

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By late 1943 when this organization saw combat, the M1 Garand Rifle would have become available. The Marines went ashore on Guadalcanal in late 1942 fighting principally with the M1903 Springfield bolt-action rifle, although some units like the 3rd Marine Defense Battalion (a rear echelon guard unit) had a significant supply of the new semi-automatic rifle. This mostly came down to the way the Marine Corps distributed their M1 Rifles when they first acquired them. Although they had been cleared for amphibious operations use in 1940 (there had been fears that the M1 Garand would be less reliable than the M1903 in these environments, which turned out to not be true). However, the Marines did not start by arming their frontline infantry units with the M1. They started with stateside guard units and worked their way down the line to ship detachments and overseas guard units. Back when rifles were personal issue and Marines would be issued the rifle during recruit training which they would then take to their unit, there are photos of Marine recruit platoons armed with a mix of M1 and M1903 Rifles. The men with M1s were bound for rear echelon units while the M1903-armed men were bound for the infantry. In fact the order would not come down to start issuing the M1 Garand to all infantry units until the summer of 1942, but this would be too late for units destined for Guadalcanal. It would not be until early to mid-1943 that Marine units, rebuilding for late 1943 operations, would get the M1 Garand.

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Other changes to armament included the Squad Leader's submachine gun (in 1942 typically an M1928/M1928A1 Thompson or the more common M50 Reising) was replaced with an M1 Rifle. Officially the Marines dumped the submachine gun from their official TO&Es from here on to simplify logistics with the new M1 Carbine coming online to replace both pistols and subguns, as well as in response to experience gained in 1942. They were still used, however, usually as carbine replacements by individual Marines are specialist weapons that were passed around a unit for certain situations such as clearing out a bunker.