Any idea as to how much of such firing a carriage could sustain? At 45 degrees, if you look at the vector mechanics, half of the force of recoil will be on the vertical axis applied towards the ground. I wonder about the carriage, wheels and axles accepting this force. The horizontal vector component translates into the distance that the carriage would recoil. There was no compensatory mechanism for the vertical component. This is the aspect of high angle fire that I find challenging. I have a hard time beleiving that such fire occured outside of controlled circumstances, such as Scharnhorsts experiments.
I have no idea about how much of such firing a carriage could sustain. But at least the French howitzers could be fired at such high angles, while the Russian unicorns could not.