So at our last session the warbands psyker repeatedly used his telekinetic control power to drop out clips of enemy weapons (by pushing the small button he assumed a lasgun has to eject/secure the power pack in the weapon). Well, first I thought that was pretty creative and let him go with it ... but after the third time I thought differently Problem also was, I ruled the dropping of the clip happened so subtly that the target still used his action to shot (of course without effect) because he could not adapt to the new situation so fast. What do you think, should I restrict this use of the power in the future? Any similar experiences?

It's a clever trick. I like it. Though maybe let the targets take awareness tests to notice this happening? Alternatively have enemies using weapons it's a little harder to do this to, like Flamers or a lot of Solid Projectile Weapons (Revolvers come to mind).

Or give enemies Rapid Reload, to mitigate this if it's really becoming a problem.

First, let the player make an Awareness check to see the switch. It is pretty small and after all it is a combat situation. Most enemies don't stand still, so the switch might very well not be seeable. Adjust the modifier for circumstances (Darkness, Fog, Smoke, etc).

Second, you might want to penalize the Focus Power roll, because it needs some really delicate usage of the power.

@ColArana: I did roll awareness for the hired lasgun but with Per 30 (or something) and without awareness skill a 10% chance is all he got . So we were playing dark pursuits and I didn't change any gear or stats of the NPCs. Maybe I should have.

@madMAEXX: I ruled that the psyker could only do it if he is familiar with a weapon. But he had Las training and had formerly been an imperial guard battle psyker so he new were to look

But yes for the future Awareness tests with appropriate modifiers are a good idea thanks.