Antietam is some of the more open ground in ACW battlefields, yet the defense relied on the use of the terrain. Some of the critical fighting occurred at a sunken road, a ready made entrenchment. 107 yards is still longer effective range than the earlier smoothbores had, and Antietam was particularly bloody, with nearly 1/3rd of the rebels falling. Smoothbore muskets often lacked rear sights...that is how limited their ranges were. 100 yards was considered the outside effective range of the smoothbores. Hence, the massed rifle fire tactics that developed. Technology guided the tactics. When ACW smoothbores were converted they often added a rear sight.

The chassepot rifle on the surface looks like an inferior arm to a number of late ACW weapons. A paper cartridge? Why? The Spencer rifle appears to be a much better weapon. Metallic cartridge...seven shot magazine. I don't doubt the chassepot had longer range, but for an american battlefield I would sacrifice extreme range for fire rate and a more weather friendly cartridge. The early war single shot breach loading Sharps rifles were very accurate...it is from them that we got the term sharpshooters. Spencer's were larger caliber and had only slightly lower muzzle velocities. Roughly 100,000 were issued to U.S. troops--I've got a list of over 60 known ACW regiments armed with them. And the Henry rifle was used by about a dozen regiments. Whatever, the case, the French didn't last very long in the Franco-Prussian War.

I don't think the popular perception of ACW lines standing on open ground blazing away is particularly representative. Yes, it happened and was part of maneuver to contact, but usually one of the lines was in some form of cover or rapidly sought some: rail fence, sunken road, stand of trees, stone wall, etc. The other line typically would stand there and trade volleys either waiting for a flanking move or preparatory to an advance or charge. Most charges/advances were repulsed (something like a dozen to one are the stats I've heard...don't know the methodology.) Maneuver under fire was in line formation, but standing erect blazing away was not something that usually lasted very long. Men were cut down too rapidly that way. After a company or two took 1/4 to 1/3rd losses they would waver and fall back. This exposed their neighboring companies who usually followed shortly thereafter.

As mentioned, Pickett's charge is a good example of how deadly open field attacks had become. It was tough to make men advance because that is exactly the sort of fate that awaited even a determined attack in most instances. At Fredricksburg and Cold Harbor the strength of rifle power was pretty clearly demonstrated. The men did indeed go forward but they were cut to pieces. Unlike TW, once a unit has taken 10 or 20% casaulties, it has expended at least half of its fighting potential. While on the surface there may be plenty of men, they are already reaching a critical state.