when you look at a video of cannons being shot during WW2 or Vietnam everybody is really busy loading and shooting and it doesn't appear that anybody is making downrange adjustments. is there a built in deflection for the shell after it is fired or are they just making the hole deeper where the shell lands?
I'm not sure your question was answered directly, but even with perfect re-alignment of the gun there is some dispersion, just like there is with a rifle, so you won't put the next round in the same hole as the last one.
Accuracy is a relative thing. On the one hand the accuracy of even an unguided artillery round, when using modern firing tables, is often more accurate than the surveying on the map, so you'll need to fire some registration rounds to work out the errors.
On the other hand with unguided artillery rounds and ranges over about 10,000 meters you'll be doing really well to land just half the rounds within 100 meters of the target.
That's not a bad thing however as like a machine gun, an artillery piece is an area weapon and having some dispersion is part of the plan. It's much more effective spreading six rounds over a 100 or 200 meter impact area than dropping five rounds in the same crater made by the first round, particularly then the kill radius of the round for troops in the open might be 30 to 50 meters (50m for a 155, an area 100 meters in diameter). The idea is to saturate the entire area with shrapnel, and as such you are engaging an area target not a point target.
Fire control computers on field artillery are a fairly recent advance that is designed to give a point fire capability when required. However, if true precision is needed there are now GPS guided rounds that are accurate enough to land inside their own kill radius. The downside is that they are expensive, so they won't be used unless that high degree of accuracy is really needed and a less expensive barrage of unguided shells won't get the job done.
The idea behind firing as a battery, is that all the rounds will arrive at the same time, providing less warning as to precisely where they will land, making all the rounds equally effective.
Some modern quick firing self propelled artillery can deliver multiple rounds on different trajectories to the same target, so that a single gun can deliver rounds that will all impact at the same time. That's a handy capability to have, particularly when the gun needs to shoot and scoot to avoid counter fire.
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As noted above some rounds are fuzed to detonate above ground as it is much more effective for antipersonnel rounds. However rounds can also be fuzed to explode on contact, or with a slight delay to allow them to penetrate before exploding. That's much more effective on buildings, bunkers, roads, runways, etc.
