[RCK]

Ben, another alternative, if you don't mind doing it, is to use a Dremel or grinding wheel, and take down enough wall on the nut driver, to accommodate the fit.  If it's a 'cheapo' driver, it's no great loss, takes about 3 minutes or less to do, and you'll be able to 'gunsmith' someone else's P99.

BTW, I'd also use some non-permanant LocTite on the screw, to make sure it stays put.