The primary focus will be on the Battle Dress (BD) worn by your basic Imperial Marine Trooper.
I'll start by addressing a few topics.
1. Armor vs. Mobility. The Imperium has gone to great expense to train this Marine, and equip him/her in very expensive Battle Dress armor. That is not even counting what it costs to transport the Marine across interstellar space. The armor should be proof against many common battlefield threats. You also want a mobile Marine that can cover ground as well get out harm's way when needed. This aspect will be highly dependent on the system design mechanics.
4. Personal Defense Weapons (PDW): Of course, the traditional weapon of the Imperial Marine is the PGMP. There are times though when you want to shoot at something and not destroy the building behind it as well. Given enough electrical power (with a MR FUSION on your back, this shouldn't be a problem), a DEI laser carbine on the forearm is a good choice. It's not that good against armored targets though, except to use as a target painting device. Another option is a mini-grenade launcher. Perhaps even shoulder mounted. As with the laser, targeting is tied in through the HUD. Loading it down with HEAP rounds would give it anti-armor capability. Smoke rounds would be useful, as long as the smoke diffused lasers enough to discourage laser painting. For unarmored opponents (besides the laser and the obvious crushing of heads), a few 30mm flechette rounds mounted along side the torso would be useful for clearing rooms quickly. Don't forget the Marine's love affair with the cutlass. Will Cold Steel be replaced by Composite laminate and Crystaliron? How about a good Space Ax? Could a nice forearm mounted, retractable spike of hull armor pierce the joints of modern Battle Dress? Shipboard fighting gets nasty, and the quarters close.
5. Man-Portable support weapons: What for? These Marine are packing Plasma Guns, and I don't mean the wimpy TNE kind! Some indirect fire might be nice now and then though. A Marine in Battle Dress should be able to mount two to four small tac missiles on his back. Either wire guided (perhaps even a camera in the nose) or smart enough to find a laser painted target (remember those forearm mounted lasers?).
6. Endurance: Off the top of my head, at least, Twentyfour hours independent action. Power and life support.
7. Medical: The suit should work to keep the Marine alive, alert, and fighting. Failing all of that, it should try to keep the Marine alive. Built in medical sensors and diagnostic computer. Autoinject painkillers, stimulants, sedatives, etc. Automatic tourniquets in the limbs would be nice.
8. Support: I see a G-Carrier per squad. Spare parts, supplies, indirect fire support, long range communications, and medical support.
Again, my main focus will be on armored Marines. Shipboard Marines are always armed and armored to the max in order to multiply the effectiveness of their small numbers.
The Tech Level of the Imperium at Year Zero is 12. Since powered battle dress arrives at TL 13, these Marines are in unpowered combat armor. No powered battle dress also means no PGMP-13. The standard issue weapon is a TL 12 Gauss rifle with an attached RAM grenade launcher. Squad level support weapons include the TL 12 Gauss SAW and PGMP-12.
The PGMP-12 I'm referring to is the one listed in Book 4 Mercenary. The weapon uses a backpack mounted power pack. 6 Kg weapon, and a 3 Kg power pack connected by a flexible power link. The power pack is good for 40 rounds. The weapon has an extreme range of 750 meters. Not a good range weapon, but what it hits while be hurt real bad. Kicks like a mule too. CT rules requires a round to recover before firing again.
The Travellers' Digest #11 goes into some more detail. This write up gives the PGMP-12 a detachable bipod. It also adds the requirement of Hydrogen fuel "H" capsules. These are magazine fed and rechargeable from a "standard" hydrogen supply.
Laser weapons should also be available. DEI lasers have the advantage of a easily replenished ammo source. They are also very effective against unarmored troops. The lack of recoil also make them a good Zero-G weapon.
On the subject of Zero-G weapons, what about our old CT friends, the Snub pistol and Accelerator rifle? These would be the weapons of choice for mid-space ship boardings (or repelling boarders). Don't forget that Space Ax either!
TL 12 also brings the Grav Belt. At Cr100,000 a pop, these are expensive. So are Imperial Marines (once you figure the cost of training, and shipping). Figure at least a squad of shipboard Marines are equipped and trained with Grav Belts. The benefits include increased mobility and increased combat load.
Back to the Combat Armor itself. I see it as a ballistic weave suit, toughened and sealed space worthy, with hard armor components. Helmet, breast & back plates, grieves, bracers, and gauntlets for starters.
Perhaps even a layer of irradiated (hardened) plastic chainmail over the ballistic weave. This would provide the extra protection against sharp edges that the BW doesn't have (an important factor in vacuum conditions).
The suit would mount some electronics, mostly in the helmet with support functions (like a power source) in the breast and back plates. Suit electronics would include a TL 12 MP sensor package (refer to Battle Dress discussion and scale down to TL 12). I still like the idea of MilSpec fiberoptic cable for secure communication.
The suit would be able to take standard PLSS units as well as hardened MilSpec PLSS units for vacuum or hostile environment situations.
Perhaps even start with a modified CbtE Suit. More data jacks. Space worthy, add pressure bands. Another option would be to start with a light vacc suit made of BW...
oversuit of light chainmail, irradated plastic, low mass, tight links covering torso, arms, legs. This is primarily to protect BW suit from rips (edged weapons, jagged metal)
chest/back plates: power supply - comm unit -PLSS links - computer - data links
forearm guards: displays & controls - retractable battle spike
gauntlets: armor plates over back of hand & fingers, as well as palm edge - in addition to being a striking weapon, placing your palms over the face plate provides an additional level of armor to protect the vulnerable faceplate.
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