Gun screw slots are often shaped a little different than regular screws. It is easy to mar them with regular screwdrivers. The best thing is to get a set of gunsmith screwdrivers - even Walmart sells a nice basic set for $15 or so.
Otherwise, take a look at how the screwdriver fits before you twist. Oftentimes a flat blade, like that in a Swiss Army knife, may fit better and flare the top of the slot less.
But don't worry if you mess them up a little - they're cheap to replace.