I always used a hacksaw to make the flathead groove
It's never a good idea to drill out screws with anything less than carbide, one of these days you'll encounter a very rare airsoft manufacturer who actually uses quality steel screws, and your drill bit will end up boring a hole 3x it's diameter lol

I've removed stuck (hehe, one dumbass friend put thread locker on his) mechbox screws before, many of them, I always went the easy route, as said above, Dremel with cut off wheel, cut a slot through the middle of the screw and use a slotted screwdriver to remove it. Since it's just the mechbox, isn't a big deal to cut a slot in the box itself (if the screw is flush) to get it deep enough into the screw to gain adequate purchase on the screw itself. Works like a charm, and is fast.

yea went to Canadian Tire and picked up

http://www.canadiantire.ca/AST/brows....jsp?locale=en

hopefully that works