Two questions for the forum here:

Firstly, there's an Assault Marine in my group. He's a killing machine, stomping Hordes, Elites, and Masters with equal ease. There's a problem, however. I recently was re-reading the rules for Horde Magnitude damage, and realized that Hordes are only supposed to take one point of Magnitude damage for every two Degrees of Success on a successful melee attack. The Assault Marine had been treating it as being one point of Magnitude Damage for every Degree of Success. We went back through his character sheet to try and find a Talent, Deed, or miscellaneous that might account for it, but to no avail. We're reasonably certain he was given something that allowed for it before, but now we can't find it. So, the question: Is there a means by which an Ultramarine Assault Marine might get one point of Magnitude damage per Degrees of Success?

Secondly, the Kill-Team is currently aboard a Space Hulk. Recently, they traveled down a hallway of the ship, open to the Warp and watched over by a malicious Pink Horror, bound to serve as a maintenance Daemon. While the Kill-Team fought off Hordes of Nurglings, the maintenance Daemon tried everything from sparks of electricity to liquid nitrogen to slamming metal doors on top of the Kill-Team. The question here is of the metal doors and Force Fields. It seems to me that a Force Field can deflect a solid attack, or else absorb energy-based attacks. A heavy metal door being neither energy-based nor particularly prone to deflection, it would seem logical (at least to me, and I admit that my knowledge of the setting is somewhat limited) that a Force Field could not then act to stop the door - or rather, it would act, and prove insufficient to the task. My players argue that the Force Field is, in fact, powerful enough to stop even a massive force. The problem I have with this is that Force Fields become godlike - a player could be rammed by a Land Raider or stomped on by a Titan, and walk away with impunity. However, as already pointed out, my knowledge of the setting is not as far-ranging as I would like. So I'll defer to the forum's wisdom here, and ask: Is a personal Force Field powerful enough to stop attacks such as the ones described above, and if not, where do you draw the line at the Force Field's power level?

Depends on how big the door is, how it is hitting the Marine, and how hard it hits. Does the door have thick teeth to ensure a correct seal and reinforce against hull breaches, or is it simply the flat side of a bay door?

depends on a few variables like those, but I would rule, provided the door is either:
A) Heavier than the Marine
B) Larger than the Marine
C) Aimed at the Marine with more force than the Astartes Weighs
That yes the force field is applicable due to their ability to absorb impact energy and refract it into light, thus its usage in combat to reflect light arms fires and glancing melee blows. However, as I said above, if enough force/momentum is in said door and it is moving at the correct speed between what the field discerns as harmless and threatening, say slower than a melee attack, but faster than 2 bodies colliding, the Force Field would then not activate.

Or perhaps the door is both larger and heavier than the Astartes and the shield does activate, but due to the amount of kinetic force it absorbs it gains a +X0 to its overload roll, and should the field prove to be unable to absorb that much damage, Overloads as per normal, and the Marine takes the remaining amount of damage directly to wounds, after TB of course.

Goddamn Lexicanum deleted the page they had on Conversion Fields (Force Fields), but the 40k wiki still has theirs thankfully:

"A Conversion Field is a moderately powerful gravitic energy field that surrounds the user who wears a Conversion Field emitter as a form of personal protection. The field's effect is to convert the kinetic energy of an impact into harmless light. When the field stops a shot, a blinding flash of light is produced as the energetic conversion occurs. The Conversion Field is normally used by important members of the Ecclesiarchy, Administratum or powerful Imperial Guard general officers as a last line of protection, particularly against assassination. The most infamous person in Imperial history who made use of a Conversion Field was the mad Ecclesiarch and High Lord of the Administratum, Goge Vandire, who used a Conversion Field to trick the Daughters of the Emperor (now known as the Sisters of Battle) into believing that he was the embodiment of the Emperor's divine will. Vandire ordered his bodyguards to shoot him, to prove he was blessed by the Emperor's holy protection and they reluctantly agreed. Unknown to both the Daughters of the Emperor and the bodyguard was that Vandire was wearing a Conversion Field and the field absorbed the laser shots that might very well have stolen Vandire's life. "

Don't think there is any talent that deals that much damage to hordes in melee. In fact, there are actually VERY few talents/abilities that deal straight magnatude damage. Those that do inflict straight magnatude damage are: Death is Joy (armour), Storm of Iron, Whirlwind of Death, Wrathful Descent (Assault), and Unrelenting Devestation (Dev). Unless I've missed one all other weapons, talents, and rules either only deal hits or do nothing special to hordes.

Lets see, possible sources for mag damage from a melee attach include:

1 point for each base ATTACK dealing 1 point or more of damage.

1 extra HIT for each successful ATTACK with a power weapon, only doing mag damage if the hit actually does one point or more of damage.

1 extra HIT (NOT MAG DAMAGE) for every 2 degrees of success from an ATTACK, only doing mag damage if the hit actually does one point or more of damage.

Double magnatude damage with the Whirlwind of Death talent (Rank 8).

Death is Joy adds 1 magnatude damage for every successful ATTACK.

Wrathful Descent deals mag damage when an Assault Marine charges a horde, but only when he is in squad mode.

Example:

Assault Marine with a power sword, Lightning attack, 50 WS, and Death is Joy armour history makes three attacks against a horde in one turn. The horde's armour and TB is less than the Assault marines minimum damage so all of his damage rolls would result in at least 1 pt of damage and thus deal 1 pt of mag damage. If the horde had enough armour or Toughness to survive the minimum roll, then you should roll the damage for each hit to see if you actually deal enough damage to cause magnatude damage.

Rolling 08, 41, and 29 on his to hit rolls:

The roll of 08 would cause 4 hits on the horde (1 for success, 2 for 2 degrees of success, and 1 for a power weapon) +1 magnatude damage from Death is Joy. Technically he should roll the damage on the 4 hits seperately but we already determined the minimum roll would deal enough damage to cause mag damage. So this attack does 5 total magnatude damage.

The roll of 41 would cause 2 hits on the horde (1 for success and 1 for the power weapon) +1 mag damage from Death is Joy. For a total of 3 magnatude damage.

The last roll of 29 would cause 3 hits on the horde (1 for success, 1 for 2 degrees of success, and 1 for the power weapon) +1 mag damage from Death is Joy. For 4 total mag damage.

This turn the assault marine dealt 12 total magnatude damage to the horde he is fighting. If the marine had Whirlwind of Death he would deal 24 total magnatude damage.

No, there is no method I'm aware of that would allow an Assault marine to get one point of mag damage per DoS on a hand to hand attack. The talents that provide bonus damage are pretty well covered by herichimo. Most likely it was simply a mis-read of the rules, happens at my place all the time!

Assault Marines will do a TON of damage to your bad guys, horde or otherwise, especially at higher ranks with better weapons. It is not hard for an AM to be able to do 30+ damage on average with a given hit- +14 or so from strength and armor, +6 from a power weapon, +2 from crushing blow is 22 in static damage before they roll a d10. Give him a power fist or the like and the damage will continue to rise, and it will multiply by 3 or 4 with their multiple attacks. This is very bad news for bad guys, and is one of the reasons it's hard to create long lasting epic fights in the game. You have to put your enemies in positions of strength, behind guns with shooters or with multiple attacks or the like. The Genestealers from The Emperor Protects, while they tend to die after 1-2 rounds, will quickly put an AM into critical with their high penetration and damage.

As for force fields, I'd rule yes the field would defend against a door - some of them anyhow - in the fluff/table top there are very few weapons that hurt the wearer and ignore the field. The description I would use is the door hits them, the force of the impact is absorbed, but the door is still there. The PC would then have to remove the door from on top of them. This is wehre their UT/US come in, as Marines based on RAW can lift and push ridiculous amounts of weight, and tossing that door aside - or at least crawling out from under it, is likely to happen.

There are certain forces in the galaxy I would rule would overcome the power of the force field- some of those would include titan or ship based weaponry, the foot of a titan (or similarly sized and weighted objects), certain weapons (shield breaker rounds of the Vindicare comes to mind first off, but there are others), and a slew of psychic powers.

Might I ask what type of field your PCs are using, and what rank/renown they're at? Iron Halo's only give you a shield 50% of the time, but you have to be Hero level renown, which is pretty hardcore. The Crozius requires the player be a Chaplain, and I'd not let a player be a chaplain at level 4 unless they were doing one hell of an RP job. The other fields I have access to in my books are mediocre at best, with their 30% or so chance to turn aside an attack, and a 10% chance of breaking.