However, since la Grande Armeť only had 139 guns in Austerlitz (Colin, La campagne de 1805 de Allemagne, 2002, Tome V, p. 134-135), the figure 50 000 seems much too high.
A quick look at wiki states that the French had 157 guns. Taking the higher number, just for ballpark calculation, that results in 318 shots per gun (50,000/157). Also according to wiki, heavy fire during the battle lasted from 7 a.m. to 4 p.m. or 9 hours; that results in 35 shots per hour per gun (318/9) or a little better than one shot every 2 minutes.
Is one shot every 2 minutes really unreasonably high?