|I'll give it a try, Zod. Union guns would have been lacing the woods with shot (falling branches, whistling hell, crashing and shouting). When the advance started, they'd have switched to shell and case, both of which are more terrifying than deadly. The advancing troops would have been treated to all of the above -- shot was especially scary when the round could be skipped over the ground and through the ranks. Again, more scary than deadly.
For cannister, I believe the maximum effective range was 200 yards, although I've read others that say 400. I'm guessing that 400 would have minimal effect. To be sure, some hits, but only against a mass such as was advancing on the 3rd of July. When the mass gets within 200 yards, the cannister was most deadly, taking out 5 or six files to the depth of 7 or 8 men in each file.
As the mass moves forward, cannister is doubled--two loads of cannister per powder charge. (I think I've read of an occasional triple-charge, but that's a desperate move for the cannoneers as the gun could as easily explode as take out a swath of the advancing men.) We're talking giant shotgun here, and rather large balls that would pass through several bodies before stopping. And the balls spread upon leaving the barrel. At, say, 50 yards, the spread of balls might reach 10 yards, more or less. As they spread in a cone, close to a quarter of them would bury themselves harmlessly in the earth. The top quarter of the circle would travel over the front rank to reach those behind, but about half the balls would be in that fatal, horizontal band -- however many men were in that 10 yards would be taken out of the battle, going back more than a couple of ranks.
Now that I've re-read your post, a double-charge of cannister would have reduced range, probably less than 100 yards, if my 200-yard limit of a single-charge is correct. A double-charge of powder would increase that range, but a double-charge with double-weight would be as risky to the gunners as to the target.
For those who haven't followed artillery: Shot is a solid ball. Shell is a hollow ball filled with powder that, upon detonation, sends fragments of the shell zinging in all directions. Case is a shell with a cluster of balls included. Cannister is very like a cluster of balls about the size of a soup can (size of can varies with the caliber of the gun) caged in a flimsy framework. The framework comes apart upon projection and the cluster spreads like pellets from a shotgun. Nothing stands in the face of, and in range of cannister.
The guns on Cemetery Hill so thinned the ranks of Pickett's men, that there simply weren't enough left to carry the line.