This is something I could technically post in the rules section but I feel its more games mastery.

Ok so In the rule book it specifically states that a reaction can only be used outside your turn and in response to some event such as an attack. It doesn't go into much more detail than that however.

Now Tech Priests can and usually do get a ballistic mechadendrite which according to the faq: “The Tech-Priest may use this mechadendrite
as his Reaction for the round or as a Half Action Attack on his own turn, but it can only be fired once per round. TheTech-Priest uses his full Ballistic Skill for the attack.”

All fair enough, for quite a while now my Tech Priest simply tanked any damage and shot back at attackers.

Where it gets tricky is that reaction and what it allows.

Picture the scene: The acolytes are fighting several mooks of a big bad. It gos on for a couple rounds and they are happily winning the battle. Cue dramatic entry of cult leader who is open to negotiation (this was during Dead Stars btw).

GM: "Suddenly a severe looking woman in fine dress enters the room from the side door flanked by several armoured guards wearing carapace, they.."

TechPriest: "I shoot her with my gun mechendrite as she enters"

GM: "You can't shoot her, shes not attacked anyone"

TechPriest: "I use my reaction to her entering the room"

GM: >.>

Much as I hate it he had a point, there is no specific limitation to reactions other than a implied use for attacks so i'm curious what other GMs allow with reactions?

I ended up ruling a reaction could only be used for something that specifically affected you, but in this case a Rapid Reaction test would allow the shot.

Probably the most irritating bit is the classic GM describing a grizzly scene or intimidating enemy and getting interupted by "I shoot it". They do listen usually I guess and are aware of the "GM talks - I listen" thing but it's easily forgotten.

I've always run it that the ballistic mechadendrite's reaction use is actually a modification to the standard uses of reactions. That is to say, PCs can (only) shoot the thing on their own turn, and they burn their next reaction for the coming turn (i.e. if you fire it, you can't dodge).

Players only get one reaction normally, so that allows it to work out.

It is interesting that you're running into a problem by using it while sticking to the normal reaction rules (e.g. you're always able to attack, so you can "simplify: social situations before the enemy can even speak).

Your problem isn't Reactions, it's your players deciding to shoot first and ask questions later.

That's fine, they should be able to do that anyway, regardless of Actions/Reactions: There's only so much the opponents can say or do on their turn, anyway. If they want to shoot, they should be allowed to shoot. Big Baddies holding long monologues in front of armed Heroes is a pretty stupid cliche anyway

The cure for your problem is to let them learn the consequences of this rather confrontational attitude. It might be self-preserving and increase odds of survival, but it is not very efficient for information retrieval purposes.

Let then gun down some important informer because they didn't take the chance of waiting to listen. Then let them stew in the problems arising from having killed their only lead. Let them fail a mission or two this way, and if they don't get the hint have their Inquisitor "question" their methods.

I've always played it as KommisarK. You may only use a reaction to defend in someone else's turn. In your turn you may give up your chance to defend for an extra shot.

I would say "I know you can use your reation (or 'I don't give a rat's ass' depending on how obstinate the player was being), but I'm still speaking which means I'm narrating so please wait until I'm finished."

You will be surprised that most players actually really respect that sort of thing and appreciate you setting boundaries.

SPOILERS

SPOILERS

Anyway, we had that encounter last session in the game I play in and if it's the big bad I think, I dual shotted her with an inferno pistol and a flametongue loaded with high density penetrators and she was only mildly annoyed.