Harry Potter and the Half-Blood Prince scored $29.5 million in its second weekend, down a steep-but-expected 62% and coming in at second place below a surprisingly strong $32 million showing for G-Force, the talking gerbils movie.

Of course, Half-Blood Prince still has not one but two aces up its sleeve.  The first is its overseas gross: in Great Britain, it still handily won the weekend and has outgrossed fellow summer behemoth Transformers 2 by a large margin (a reversal of the U.S. grosses).

The second is IMAX: Half-Blood Prince could only book a limited number of IMAX screens when it opened because Transformers 2 had booked a month-long exclusive run.  That time frame is winding down, so when Harry takes its place, he might get a second wind.

Thanks to its huge opening, Half-Blood Prince is still running a few percentage points ahead of Order of the Phoenix, which made $290 million.  With a little help from IMAX, Half-Blood Prince could be the first Potter film since Sorcerer’s Stone to make over $300 million…but we’ll have to wait and see.