Come up with a position where both players are forced to capture for the next N plies (N half-moves). Winner is the entrant with the greatest N. It must be possible to arrive at your position with a sequence of legal moves from the starting position.
Here is my entry with N=16. White to move:
Bill Smythe
Couldn’t there also be a White bishop somewhere on the h1-a8 diagonal?
Alex Relyea
Probably, but that wouldn’t increase N, as far as I can see.
EDIT: Or did you mean the g1-a7 diagonal? See Jeff’s comment below.
Bill Smythe
Moving the f2 bishop to e3 and putting a white bishop on f2 is possible.
N=19
Can you put the Ra8 on Ra3 and do something with the a7 square?
Lots of promoted pieces for both sides: might need a proof game to establish that the position is legal.
Bin Laden, meh: this is more interesting.
I considered black Ra3, white Qa7 and a rook on d8, but that rook is not guaranteed to be the last capturing piece on d4, and an earlier capture breaks the force sequence.
Most of the promotions are to queens with only a few limited to bishops.
It looks like there would need to be four captures on each side and only one rook and one knight is missing from each side (changing it to white knight on h6, white bishop on g8, black knight on a3, black bishop in b1 adds a capturable rook to each side). One more bishop may need to be removed to get the final capture needed (reducing N to 18 but also opening a diagonal for the checking queen to have gotten there).