Yeah, there's a lot of math behind distance shooting. Tons of variables to consider.

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.

> Why can't you have the situational awareness that nobody is there just by looking with your eyes?

The entire setup of an army, all its communication mechanisms, and systems of control and safety measures, are focused on telling you about what is happening forward along the axis.

Trying to figure out if it's safe to fire backwards is going to be massively error-prone as nobody is set up to tell you this and asking it is going to cause massive confusion.

Also, you can hit things that you can't see, with ricochet and fall of shot.

Because that only allows for you to sight in at that range.

Unless that range also exactly matches the range to the enemy targets, from your current nest, you won't be properly sighted in for your intended targets.

The ranges over which snipers make kills is sufficient that even a very small miss-alignment of the sights translates to a miss (often by many yards) at the target. According to Wikipedia [1], the sniper record is 3,540m (3,871 yd). That is 3.5km - imagine targeting something 3.5km away, firing, and having your shot hit the target.

Even in 1916, while sniper's may not have operated at 3.5km distances, they could easily have been operating across 500-1000m distances (the Wikipedia article lists a 1918 record of 1,280m (1,400 yd) or 1.2km). Even at those relatively shorter ranges, the margin for misalignment is still tiny. You have to actually sight the rifle in for the actual range, the actual elevation (whether you are shooting uphill or downhill) the current wind conditions, etc. And still hit a rather small target. Aligning to some other fixed berm will not be nearly accurate enough.

[1] https://en.wikipedia.org/wiki/Longest_recorded_sniper_kills

> Why not have a berm in the rear area that's designated for sighting in rifles?

Because they're using fixed (non-floating) barrels and they need to re-zero in the field and can't get back to a rear area to do that.