I'm not sure why your weapons weren't working. Could be something to do with damage type immunity. What sort of bullets were in your sling?
As for improving your performance, what sort of graphics card do you have? BG2 has issues with some graphics cards when running in 3D mode. Check the readme file to find out which cards are happy with it and which aren't. If you have a low-end card, it's probably best to stick with 2D mode.
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"Our deepest fear is not that we are inadequate.
Our deepest fear is that we are powerful beyond measure."