That was my impression that high angle fire was not used in the period yet perpectuated in many of the books of the last 50 years. The designs of the carriages as you have stated before precluded this. The introduction of the vertical elevating screw in 1792 to the Gribeauval Howitzer meant this was the last of the howitzers that could do high angle fire was now limited to 25 degrees or so. This would have less strain upon the carriage. High angle fire due to the direction of the forces would break a carriage as is recorded. An interesting occurence is the 8in howitzer carriages broken in the Peninsular as discussed by Dickson.