Oh I've read Paddy Griffith's book too.
There's excerpt after excerpt of how the Union artillery tore gaps in the Confederate front as it advanced... Which no doubt caused not just physical loss but it probably wore on the troops mentally.
Well, "tore gaps" is probably overselling it at most ranges - we shouldn't allow the evocative language to overcome what actually happened when cannister and shot were fired, which was that canister fired 27 balls in a tight cluster; shot fired a single iron cannonball which could hit up to two people when directed at a two-deep line seen head on.
At optimum range canister can hit up to 8-10 people (4-5 files, at 100 yards where the shot cone is at the ideal dispersion - spread out enough to hit multiple files but still dense enough to blanket the area - and elevation isn't an issue), but most shoots were not optimum; this is why there were 33,000 Union artillery fires at Gettysburg (as well as a couple of million million small arms rounds) and about 23,000 Confederate casualties (from all causes including captured-unwounded). On average each Confederate casualty took 1.3 artillery fires and ~100 small arms rounds, so it should be immediately apparent that if we look at the artillery alone we should expect about one casualty for every 2-3 Union artillery rounds.
If we assume that all of the guns from Daniels down to Ames (45) fire canister at ideal dispersion range then they might cause as many as 400 casualties, but they'll only actually get one shoot each at that level of effectiveness - at 300 yards and the correct range setting the density reduces the effectiveness of the fire by half or more, while at 50 yards the reduced spread of the cone also reduces the density by half. And if you have the range setting wrong then cannister at 300 yards just kicks up dirt ahead of the advancing men.
This is absolutely enough to cause a single brigade to stop, assuming this idealized situation happens, but then again it is the full artillery of two army corps - and if Stannard wheels out of line by as little as 100 yards (i.e. he has a front of about 300 men enfilading Pickett's main charge) then it means that the Confederates don't have to enter that zone of maximum cannister effectiveness to enfilade
him.
It's also worth pointing out that if 10% of the artillery fires at Gettysburg were cannister and they hit an average of 4 men each (50% of theoretical maximum effectiveness) they'd still account for more than half the total Confederate casualties - which means either everything else was basically useless at causing casualties or the cannister wasn't as effective as that.
Per 67th on the spread effect of cannister:
The cone of dispersion is such that at 100 yards almost all the balls are in a elongated circle about 11 feet in diameter, with a few random balls knocked off course falling outside this (extreme spread is about 32 feet diameter, but about 90% of balls are in the inner cone).
If the troops are 6 ft tall, and packed shoulder to shoulder then about 30% of the balls go over or under the target. The effect on a line is lethal, with whole files blasted away.
At 200 yards, if the magnus effect is ignored the inner cone has opened to 22 feet then 83% of balls go over or under. This overestimates the effect due to the magnus effect displacing the balls. Suddenly rather than concentrated violence of knocking a hole in the line the effect is reduced to a few men hit scattered throughout a company.
At 3-400 yards the balls are so dispersed that hits become very unlikely indeed, assuming you aimed correctly. At 350 yards the correct elevation is about 1.5 degrees. Being 0.25 degrees off in either direction will send the whole blast over or under the target.
So the high lethality of canister is very much a thing of the 100 yard range window specifically, which is to say musket range. Interestingly this means that if soldiers in the Civil War actually
did use their rifles as rifles as a matter of course then canister's effectiveness would be drastically reduced as the attacking soldiers could engage and disable the guns from outside canister range...