I've been giving this topic quite a bit of thought since the thread was first started, and I've finally got around to putting some stuff down on paper. So, here's my take on the Marines and the army (still a work in progress, but the bulk of it's done).
Thanks to the various posters in this thread who have inspired several parts of what is to follow.
IMPERIAL MARINES
The Imperial Marines are the elite troops of the Imperium. They receive the best training and equipment available, and form the forefront of any major operation. They are also best equipped and placed to deal swiftly and effectively with sudden threats and emergencies. However, they rely on the Imperial Navy for logistical support, and are generally ill-suited to long or middle-term deployments – protracted operations are the domain of the Imperial Army or local forces.
Imperial Marine HQ has five subordinate commands – Ground Command, Star Command, Security Command, Adminstration Command and Combat Command.
Ground Command is tasked with developing training and doctrine for ground assault elements of the Marines, while Star Command peforms the same role with respect to fleet security and space assault troops. Security Command is responsible for Marine elements deployed as security personel for embassies, fixed naval assets and other political assets. All actual Marine combat assets fall under the control of Combat Command. Administration Command operates at the Domain level, and deals primarily with personnel management, including recruiting, pay, senior promotions and posting. Most other logistical tasks, including the provision of dedicated medical personnel and aerospace fighter support, are the responsibility of the navy.
Combat Command can be generally divided into two categories – Marine Response Units and Fleet Marine Personnel.
Fleet Marine Personnel are allocated to almost all Imperial Naval vessels. Corvettes and frigates generally count a single Force Lieutenant or First Sergeant as Marine Officer, who is in charge of ship security. Destroyers are allocated a Star Marine section of six personell, commanded by a Force Lieutenant, while battleships have a Star Marine platoon complement, commanded by a Force Lieutenant or Force Captain.
Cruisers, being more wide-ranging vessels designed to respond rapidly to emerging threats, are allocated larger Marine complements that include ground assault elements. These Marines are not merely present for security or to conduct boarding actions, but can also be called upon to undertake ground actions.
Light cruisers carry a Variable Action Company consisting of one platoon of Star Marines, two ground assault platoons, a support platoon and a grav tank. Heavy cruisers carry two such companies, while battlecruisers carry an entire Marine Fleet Readiness Unit. A small number of Marine auxillary cruisers are assigned directly to the Imperial fleet, and carry a reinforced Variable Action Company.
Marine Response Units are not allocated to regular navy vessels, but are instead based on their own fixed facilities. Each MRU is generally allocated three Marine auxillary cruisers, three escort cruisers and an escort carrier, and the unit can be rapidly deployed to deal with any threat or emergency in the local region. MRUs are generally utilised in situations where troops are needed, but a conventional naval presence is either unavailable or unnecessary.
Ground Assault Section (10)
- Fire Team [2 x Marine, LCpl, Cpl; FGMP15, MRFGG, AGP, 2x Heavy Gauss Rifle or CC Pulse Laser Rifle]
- Fire Team [2 x Marine, LCpl, Sgt; FGMP15, MRFGG, AGP, 2x Heavy Gauss Rifle or CC Pulse Laser Rifle]
- Grav Carrier (Drop Capable) [Driver, Gunner; LFCG15, Triple Heavy Laser-15 or ARFM]
Ground Assault Platoon (36)
- 3 Ground Assault Sections
- Command Section [SSgt, FLt/FEns, Cpl, Marine, Grav Carrier, Driver, Gunner]
Star Marine Section (6)
- Fire Team [2x Marine, Cpl; 3x CC Pulse Laser Carbine]
- Fire Team [2x Marine, Sgt; 3x CC Pulse Laser Carbine]
Star Marine Platoon (28)
- 4 Star Marine Sections
- Command Section
Variable Action Company (~200)
Star Marine Platoon
- 2x Star Marine Section [Battle Dress]
- 2x Star Marine Section [Combat Armour]
- Command Section [Battle Dress]
2x Ground Assault Platoon
- 1x Ground Assault Section [Battle Dress]
- 2x Ground Assault Sections [Combat Armour]
- Command Section [Battle Dress]
Assault Boarding Vehicle Platoon
Support Platoon
- Fire Support Section
- EW Section
- Recon Section
- Technical Section
- Assault Engineering Section
- Signals Section
Grav Tank
HQ Section
The VAC(reinf) gains an extra ground assault platoon, and the grav tank is moved into a seperate armoured platoon containing three tanks. Two or three VACs may be combined into a Variable Action Battalion, under the command of a Lieutenant Colonel.
Marine Response Unit/Fleet Marine Readiness Unit (~530)
Star Marine Company
- 1x Star Marine Platoon [Battle Dress]
- 1x Star Marine Platoon [Mixed]
- 1x Star Marine Platoon [Combat Armour]
- HQ Section
2x Ground Assault Company
- 1x Ground Assault Platoon [Battle Dress]
- 2x Ground Assault Platoon [Combat Armour]
Armoured Platoon [3x Grav Tank]
HQ Sect [FCmd]
Support Company
- Meson Battery [3x Meson Sled]
- Fire Support Platoon
- Assault Boarding Vehicle Platoon
- Assault Engineering Platoon
- EW Platoon
- Signals Platoon
- Technical Platoon
- Recon Platoon
- HQ Section
Admin Platoon
HQ Sect
Two to four MRUs and/or FMRUs can be combined to form a Marine Fleet Element, under the command of a colonel. Larger marine formations exist only for the purposes of chain of command and logistical responsility – a threat that cannot be swiftly dealt with by the incredible firepower of a MFE and it's requisite naval support inevitably means a battle of attrition, which falls under the umbrella of the Imperial Army's mandate.
Subsector forces form a Marine Brigade, under the command of a brigadier, whose main job is to allocate resources and plan deployments, but who subordinates operational command to local Marine, naval or political commanders. Generals hold command positions at the Sector and Domain levels, but at this point they are fully integrated into the Admirality and answer directly to senior naval personnel. During full scale war, a brigadier or general may be posted as OPCOM MFT (Operational Commander of Marine Forces in Theatre).
Non-Imperial Marine Forces
Most navies maintain a cadre of marines as ships troops, although ship's troops is almost all they ever are. Responsible for ship-board security, soft boarding, and sometimes hard boarding, they generally lack the equipment and training of Imperial Marines. Naval infantry are also sometimes employed, generally for the defence of planetside naval installations; a small number of navies maintain an infantry force capable of limited offensive operations. Some highly mobile army units have similar capabilities to the Marines, and some of these are called marines locally; to a Marine, however, they're naval infantry or army, not Marines.