So this applies across most if not all of the 40krpgs, but because the question came up in a Rogue Trader game initially, I'll post it here.

When a PC has a force field of some sort, do they resolve the chance that it will block the attack on them before or after they roll their reaction?

The rules for fields that I've been able to find do not address this directly. The closest I've been able to come after some in-depth discussion with another RT GM friend of mine, points to the following language in both the field and the reaction text blocks:

Force fields activate, "when a character wearing an active field is attacked , roll 1d100, [...]", while the entry on Dodge says, " after a character is hit , but before Damage is rolled, the character can attempt to avoid the attack by [...]"

So the interpretation of reading those two rules in conjunction suggests that you resolve the field activation first, and then and only then if the attack is successful and damage should be rolled, does the dodge roll come into play.

However a couple of my players are arguing that logically from an in-character point of view, someone is going to dodge as soon as they realise they're being attacked - even with a force field, as the potential for that to activate is not a 100% certainty. So they're arguing that you should resolve a dodge action first, then if you don't manage to avoid the attack in the first place then you should see if the field activates, kind of a Hail Mary second chance situation. I have seen a wiki from another GM's RT campaign that says they houseruled this in as a rule.

I'm stuck between both viewpoints, because mechanically I can see how the language used in the system supports view a - fields come first. But from a fluff/storytelling/logical character actions perspective, I also can see the validity in my players' view b - resolve reactions, then see if the field triggers if you can't mitigate the attack in another fashion.

Thoughts? Suggestions? Ideally feedback from an official source would be great, but right now I'd just take logical arguments for either side - or a link to anywhere that this discussion has been held before if it's available.

The bottom line I would enforce is that the combat system is an "abstraction"; the GM can twist logic to an extent in the name of game balance. In hands of an enemy, tying Force Fields and Evasions together makes her easier to hit, but players will also have reduced benefits from Force Fields. I prefer to divorce Force Fields and Evasion, so that overwhelming an enemy's defenses requires burning through their Reactions AND Overloading their shields.

The way I play it is thus.

The force field only activates if you would be hit.

If you are missed, either due to dodging, cover, or the attacker missing, then the force field doesn't activate nor is there any chance of it overloading unless the force field is actually hit by something.

To me, this is the method that makes sense in terms of good rules logic and storytelling.

As per our skype conversation.

Page 168 of the Dark Heresy 2nd Edition Core.

I'd vote in favor of players, so roll Fields first, since I believe you only get one, or maybe only so many, depending on what you are playing/have purchased Reactions, while Fields can trigger repeatedly. Maybe "I don't care how fast you THINK you are, but if I shoot you, you are hit before it totally registers, while the Field would slow that down a smidge, and the flicker would clue you in to dodge aside." You might respond then that with no Field, NO ONE would be able to Dodge, and yeah, i can see that in my own words, but I'd still vote for Field first, to keep the more limited Reactions in reserve. That's just me, of course.

Rolling for fields before evasion makes little sense to me. So I insist that reactions (if any) are tried first.

Reactions come first. Unless force fields are also time machines which take you back in time to before the point in which they failed to block an attack.

I don't really like logic in this instance. Logically, the field activates or it doesn't, regardless whether the person dodged, parried, or performed a deer-in-the-headlight maneuver. It doesn't matter which is rolled first. The reaction is used up AND the shield has to roll for malfunction. It's lose-lose for the player.

I've always had the shield roll last, simply to reduce the amount of die-rolling. I could always change, though.

Reactions first IMO. You shields are not going to do anything if you dont get hit.

That's a very questionable argument. If the bullet/beam/missile/whatever misses by an inch, it still misses. Does that mean the shield didn't activate?

Suppose that depends on how close you think the shield is to the person. In any case, the system doesn't really allow for the level of granularity to determine exactly that.