Hey so pretty much I have a player who is a Chaos sorcerer and the players got into combat.

He did a doombolt (half action) & shot his bolt pistol (half action). One of my players objected saying that it is two attacks and it shouldn't be allowed according to the rules.

But I couldnt find anything in there so I've been allowing it ever since.

Is there an actual rule saying you can't do it or is it OP and I shouldn't allow it?

You can't do two actions with the same subtype. The player that objected is correct, since both Doombolt and a Standard Attack have the Attack subtype. You could use psychic powers that don't have the attack subtype with a bolt pistol though.

Pg 234 has the rules outlining subtypes. More specifically, you can only take one action with the attack subtype and one action with the concentration subtype during your turn. Do note, however, that you can take more via reactions such as counter-attack, since that's not your turn.

So in this case, yes, it was against the rules because doombolt has the attack subtype, as does firing a gun. However, this is not always the case. For example, your player could have used a free action to activate precognitive strike (which has no subtypes) prior to firing his gun, and then spent his other half action activating telekinetic shield, which only has the concentration subtype.

However, had he doombolted instead of fired his gun, he could not activate telekinetic shield, since Doombolt has both the attack and concentration subtypes. Which subtype an action and psychic power has is listed under their action description in the combat chapter or under the power itself, respectively.

Edit: Annnnd Ninja'd