Congratulations.
I doubt if Reynolds was killed by a sharpshooter. It is more likely the bullet was fired by one of Archer's men who overshot at the Iron Brigade and struck Reynolds instead. As for Sedgwick, I don't think Powell slew him. Powell wrote, "I raised my sights and took the chance, and sir, he tumbled right off his horse." Charles Grace may have mentioned shooting an officer off his horse that day too. Sedgwick was on foot when he was slain (see statement of 16th Maine Adj. Albert Small) and this would refute either claim. Absent forensic evidence (like recovery of a sharpshooter bullet), we don't know exactly who fired the fatal bullet.
Re: Sumter, yankees were hit on Oct. 9 and 19th too. Sumter had been reduced to an infantry outpost and relied on other forts to defend it from ironclads. The distance from Sumter to Morris Island was 1390 yards and Confederates atop the ruins of Sumter could observe the Union soldiers working at (former Confederate) Battery Gregg which was on the northern end of Morris Island; facing Sumter's "Gorge" wall. Some distance may be added to the 1390 yards too because it is unlikely that any Union soldier would rest on the beach. It was too dangerous.
You may want to contact
The Company of Military Historians and ask the Administrator Dave Sullivan for the article,
Sharpshooters as Prisoners. The article examines one soldier's statement that went, "[Sharpshooters] are not likely often to be taken prisoners, as death is considered their just penalty; for as they are very seldom in a position to show mercy, so, in like manner, is mercy rarely shown to them." In fact, I encourage you to join as you seem to be interested in military matters. Its membership is composed of historians, curators, conservators, collectors and ordinary people who have an interest in military history. Either I (and I'm sure CivilWarTalk member Major Bill) will be happy to sponsor you.
Sniper was coined by the English in the 18th Century but the word did not enter common usage until the Boer War. While I've seen it used by Civil War soldiers, they either penned their works after WW I popularized the term or
it cannot be determined when they wrote it or whether their editor replaced "sharpshooter" with "sniper." You may want to read the sidebar on page 117 of
Sharpshooters: Marksmen Through the Ages.
Last, I wish I had written such an awesome paper when I was your age. If you can land a job with the National Park Service as a park ranger, do it! Get paid to have fun. Keep up the good work and remember, the learn'n never ends.