Not really a crossbow, but what I hope is interesting information on the effectiveness of a somewhat similar weapon, the Roman scorpio - a torsion powered "bolt thrower":
http://www.planet-schule.de/sf/php/0...p?sendung=8624 (jump to minute 11:00, in German)
The testing took place at the Fraunhofer Institute for High-Speed Dynamics (
http://www.en.emi.fraunhofer.de/).
With the weapon in the video above we (legio8augusta.de) managed a maximum of 12 bolts fired in 60 secs with a 3 man crew after some hours of scorpio drill over 2 days and a few adjustments on the weapon.
Best,
Martin