I haven't got the sizes to hand mate (I'm at work at the mo) but you need TWO drills - one the size of the screw shank (the threaded part less the width of the actual thread) and a slightly larger one the same size as the unthreaded section of the screw. If you try doing it with one drill you are likely to either split the stock or find that the stud won't hold firmly - errr, dropped rifle.
I hope this helps.
PS Make sure the stock is firmly held (padded Workmate or similar) and use a bit of tape to stop the bit skidding.