I doubt they were judging distance to the exact , But even at 200 yards the first volleys would have been deadly to massed infantry in line and advancing we have eye witness accounts of the devastation caused the fact that a small percentage actually made it to any part of Union lines was a miracle in itself.
Its also impossible to say what the hit rate was in any battle but what we do know is the vast number of casualties sustained in the ACW were by Mini Ball we also know the ACW was most bloody war in American History and the Rifled Musket and out dated Tactics was the cause of that imho and in a lot of people's opinion.
In the first place, if they weren't judging distance that precisely
they'd have missed. This is the whole issue with small arms fire using a Minie rifle at long range - if you get the range estimation wrong you miss. At short range ("point blank" range, around 100-150 yards) you just point the gun level and hit.
We know how many men became casualties at Gettysburg (about 50,000 from all causes, including prisoners and the artillery) and we have an estimate of how many bullets were fired (about 7 million). What this means is that, if artillery did
nothing, one in 140 bullets fired actually caused a casualty.
So we do know the hit rate at Gettysburg as a whole was no better than about 1 in 150.
Now, as for the issue of "even at 200 yards the first volleys would have been deadly" - remember, the Confederates advanced to within 100 yards, engaged in a firefight for close to half an hour, and then retreated. About half of them retreated, and some of the men who did not retreat were captured rather than WIA/KIA.
I think it's more likely that most of those casualties happened in the 20+ minutes of firefight, rather than the one volley from two regiments which happened at longer than 100 yards range, simply because any other conclusion is basically impossible.
And as for your point about "a small percentage actually made it to any part of Union lines was a miracle in itself" - well, that's what I was saying, isn't it? Most of the men were not killed by the first volley - they slowed down, fired back, and got settled into a firefight. If they had kept going then they would have taken about three volleys before reaching the Union position (it's impossible for them to have taken any more, the weapons do not fire fast enough) and would have stormed the position - much like Emory Upton's men stormed the position they were launched at, because they did not stop.
Finally I'd like to point something out about the WW1 tactics comparison.
You see, I'd like to compare some situations.
With the Napoleonic war, you have smoothbores able to be fired five times a minute in an emergency which can hit targets reliably out to 100 yards if aimed correctly. Men advancing from 100 yards to close range over the course of a minute will take five close range volleys, and from 100 defending men that will mean they have 500 bullets fired at them - at an accuracy rate of about 1 in 100 to 1 in 200 (typical values for a whole battle, in the same way the Gettysburg number was calculated), it means that those 100 men will cause about 3-5 casualties on the attackers.
So to recap:
Napoleonic: 100 men means 500 bullets and 3-5 casualties.
With the Civil War, you have muzzle loading rifles able to be fired no more than twice a minute (Minie rifles were slower loading than smoothbores because the bullets fitted more tightly) and which at Gettysburg can hit targets out to - well, let's say 200 yards, even though most regiments opened fire at a shorter range. Men advancing from 200 yards to close range over the course of two minutes will take four volleys, and from 100 defending men that means they will have 400 bullets fired at them - if we assume an accuracy rate of 1 in 80 (which is twice as good as the real Gettysburg whole-battle value) it means those 100 men will cause about 5 casualties on the attackers.
Gettysburg: 100 men means 400 bullets and 5 casualties.
At the Battle of the Inkerman in the Crimea, you have Minie rifles being fired out to 600 yards effectively, and at that battle we
know the hit rate was at least 1 in 18 (because we have the Russian records of how many men were wounded by Minie balls). Men advancing from 600 yards to close range over the course of six minutes will take 12 volleys, and from 100 defending men that means they will have 1200 bullets fired at them - which means they will suffer about 60 casualties.
Inkerman: 100 men means 1200 bullets and 60 casualties.
In the First World War, you have magazine rifles able to fire twenty aimed shots a minute, plus the company machine gun which has about 450 rounds per minute. They can engage the enemy at long range and with high accuracy, but for now we'll assume the hit rate is 1 in 80 (no better than Gettysburg) and that they can engage the enemy at 300 yards (in reality it would be more than this).
In 3 minutes the 100 men will generate 3 x 100 x 20 = 6,000 shots, plus another 1,350 from the company machine gun, for a little over 7,200 rounds. Even at Gettysburg hit rates, this will generate about 90 casualties.
WW1: 100 men means 7,200 bullets and 90 casualties.
What you're effectively arguing is that
Gettysburg is closer to
WW1 than it is to
Napoleonic. I don't think there's the evidence to support this; if the accuracy stats were as good as they were for
Inkerman then I'd perhaps agree with you, but they are
verifiably not - if Pickett's Charge had gone up against men with a 1 in 18 hit rate, then over the course of the 20 minutes of the firefight every single one of them would have died twice.