The main part of the armour is the massive powered torso which encloses the chest and arms. Beneath the armoured chest plate coiled energy cables transmit power into the arms, effectively multiplying the wearer's fighting abilities three or four times over. During this period most fighting consisted of close combat, warriors preferring to grapple with each other rather than use long range weapons - the power of a warrior's chest and arms was therefore of paramount importance.
Space Marine Armour by Rick Priestley (White Dwarf 129)
Power Armor has bad rules. It has a good save, but that means units that wear it have to pay a lot and get too little out of it.
Power armor sort of has implied rules for strength – Sisters and
PA marines can carry vehicle grade weapons the way people carry rifles.
Other infantry cannot carry anything as heavy as a multi-melta. Guard use teams of two, Orks use less powerful weapons like rokkits, and even the dark lance is not a tank gun, since if the dark eldar had tanks they would use something more like a pulse laser or prism cannon.
Even marines without armor do not carry these heaviest weapons, so the ability comes from the reinforcement of wearing armor.
Power armor does not have any rules for being better at using smaller weapons or fighting hand to hand.
Power armor originally had exactly the same save as carapace armor, except that carapace was so heavy it forced a movement penalty, and power armor did not.
The only actual rule power armor has is for a 3+ armor save.
This is bad, for gaming reasons.
Models with power armor are usually supposed to be very lethal. However, so many of their points go into their very good save that lethality in shooting is a miniscule part of their identities. Tactical squads are, point for point, less lethal at shooting than shoota boys.
Some models, like veteran marines and Death Guard, are meant to be very good at using small arms. However, since basic PA models already spend so many points on armor, leadership, and close combat, the more elite versions have the same shooting ability but vastly better everything else, and so their ability to use a bolter is even more eclipsed.
Sternguard are veterans, but the only reasons their shooting attacks are good is their equipment, not their abilities.
This is bad, for background reasons.
Autocannons are used for shooting materiel like airplane armor and light vehicles. This is exactly the weight of wearable armor that makes PA. When the predator first came out, its autocannon destroyed marines, because it is a tank gun.
Plasma missiles are missiles plasma, but do not penetrate power armor.
Power armor should not resist Heavy Bolters.
Normal armor is supposed to resist small arms and shrapnel, but not machine guns. That’s why flak armor resists lasfire, shoota rounds, eldar lasfire, and frag grenades. That is also why flak armor does not resist machineguns: big shootas, shuriken cannons, and heavy stubbers.
Heavy Bolters are not the human machine guns, heavy stubbers are. Heavy Bolters are the machine guns for power-armor wearing warriors, so they should penetrate power armor.
Ork rokkits are anti-tank rockets, for pete’s sake.
Marines would be better if power armor were weaker. Examine:
Power armor affords it wearer a 4+ save. Due its comprehensive coverage, models wearing power armor may add one to the result of all armor save rolls (so against bolters, shootas, lasguns, etc. it is still a 3+ save). Models wearing power armor may fire weapons of strength four or below one more time than usual, so a bolt pistol would roll two dice to hit, and a bolter would roll a maximum of three dice to hit, three to wound, etcetera. This does not apply to template or blast weapons.
This would mean: predators are good. Basic Marines and Sisters are good at killing things, instead of lame. They also have to put themselves in close range, where they can immediately destroy heavy weapon crews, instead of far away, where they can be themselves destroyed by heavy weapons. Guardsmen may not pack so much plasma. Gryphon Mortars are better, so Earthshaker Cannons may be less common, so Terminators are safer. Veteran marines do not have to rely so much on extra wargear to be useful. Storm Shields can be less silly, since they can be changed to a 4+ invulnerable and still be useful.
Storm Bolters can be less silly. At the moment, they are assault two, which means that even though they are twice as large and heavy as a bolter, a guardsmen can wave one around more easily than a bolter. The reason they are assault two is that third edition
TDA had no version of relentless. If
TDA has the same rules for shooting bolters as power armor, then storm bolters can switch to just combi bolters, and the two effects combined mathematically double the shooting power of a Terminator with a storm bolter. Examine
Terminator or Tactical Dreadnought Armor affords its wearer a 2+ save and makes the model
relentless. When firing weapons of strength four or less, it follows the same rules as power armor. Models wearing
TDA may treat bolters, storm bolters, and combi-weapons as though they were close combat weapons. They also may take a 5+ invulnerable save.
Storm Shields afford their bearers invulnerable saves on a
d6 roll of 4 or better.
You now have models that can erase any moderately armored infantry they get within 12" of, and that are hard to kill with small arms or close combat attacks, and are not any more expensive, and perhaps less expensive, than ones regular 3+ power armor. They also have to get close to the enemy, where they are more efficient at killing than the enemy is at killing them, which is the entire point of shock infantry.
