Most artillery pieces, even today, can be depressed to about -2/-3 degrees and fired 'over open sights'. I think it was the Aussies who were known to use 25-pounders as AT guns during WWII, exploiting the direct-fire capability.
Of course, if you need to fire over open sights, it means you're in serious trouble, but the option is still there should the worst come to the worst...