The civil war is often credited as being the first modern war. One of the key differences in warfare before and after the modern warfare was the efficiency of weaponry. Did that efficiency lead to higher casualty rates in short time spans?
I have to correct your numbers, here; the Union army suffered about 1800-1900 dead in this battle and about 9000 wounded. This is for the entire battle, which lasted 12 days. The famous June 3 assault accounted for more than half of all Union casualties, and was horrifically bloody, but we don't come close to 7000 dead.
As far as your actual question, the weapons and tactics used in the Civil War were really no more advanced than the Napoleonic Wars. Repeating and breech-loading rifles did begin to appear in larger numbers in the last few years of the war, but were never present in sufficient numbers to make a difference in casualty rates.
Compare a bloody Civil War battle to a bloody Napoleonic battle:
At the Battle of Gettysburg, around 175,000 total men fought in the battle, of which about 51,000 became casualties over three days. You have a total casualty rate of 30%.
Now let's look at the Battle of Austerlitz. Of 152,000 total men, 45,000 became casualties, or an almost identical 30% or so casualty rate. But, Austerlitz was fought all on one day, so those 45,000 men were captured, killed, or wounded much more quickly than the 51,000 at Gettysburg.