You need to keep in mind the distances here. For calibrating your rifle you're shooting at targets several hundred yards away, making it much harder to tell if the area is clear. This also isn't happening on a football field, the area around the trenches is going to be scored with secondary trench lines, foxholes, communications trenches, big craters from artillery, etc. And the people in there aren't exactly trying to make themselves known, they are going to keep low and you may not even be visible if they are in a little offshoot of a trench or something. The secondary lines are also going to be relatively densely populated. Odds are if you try to shoot at a target a couple hundred yards behind the trench lines, the round is going to be within a dozen yards of someone's head. You probably wouldn't hit anyone, but if someone stands up at the wrong moment... an even bigger danger is if your shot goes high. If your shoot goes above the rock you're aiming for, it will keep going further back into the lines. Given that this would be 30 caliber ammo and there's no body armor, that bullet would still be going fast enough to kill someone 1.5 miles out, maybe 2 miles. Someone very familiar with the area and with good situational awareness might be able to guarantee there's no one between them and the target, but not for a mile out.
The other big factor is what your buddies in the trench will think of this arrangement. If they know it's you shooting over their heads they will be absolutely pissed, and probably face some sort of discipline from officers. If they DON'T know it's you shooting in their general direction (which they probably won't unless you make sure to tell everyone in advance) it will probably prompt return fire. If they can tell from the sound or muzzle flash that the shot came from their trenches, they may think a sniper infiltrated their lines, which will merit a particularly aggressive response.
I wouldn't be surprised if someone, somewhere during the war did this, but it certainly wouldn't be condoned.
The other big factor is what your buddies in the trench will think of this arrangement. If they know it's you shooting over their heads they will be absolutely pissed, and probably face some sort of discipline from officers. If they DON'T know it's you shooting in their general direction (which they probably won't unless you make sure to tell everyone in advance) it will probably prompt return fire. If they can tell from the sound or muzzle flash that the shot came from their trenches, they may think a sniper infiltrated their lines, which will merit a particularly aggressive response.
I wouldn't be surprised if someone, somewhere during the war did this, but it certainly wouldn't be condoned.