Things have changed a bit. You’re in the heart of Antietam and it's time for some heavy combat. Join the Union forces running down the road and approach the Union camp. You’ll get sucked into a firing line. Thankfully you’re in the middle and generally safe from the enemy for the time being. Just enjoy the computer-controlled run through the cornfield until you make it to the dividing irrigation ditch and the Confederate line. You’re frozen in place at the moment.
You do have a Henry rifle with 30 bullets. We’re going to save those for the next little fight though. It’s also a lot more fun to play along with the firing line and stick to the musket. You should be fine, especially since your goal isn't to break the Confederate line right now. You just need to live. Your goal is to keep firing at the Confederates. If you pick them off at a steady rate, then enough of the Union soldiers will survive to act as a shield. You can’t win this fight. You only need to survive long enough for the main wave of Confederate reinforcements to break the Union line. As soon as you regain control, you need to drop into the irrigation ditch and sprint down to the objective marker. If you move quickly, then you should survive.
Join up with the few soldiers by the rocks ahead. Take up a position behind the biggest rock and wait for the Confederates to come around the bend. There should just be 4 soldiers and an officer. Go ahead and pull out the Henry rifle. We’ll get some fresh ammo for it soon anyway. Take aim and kill the officer first. He’s the biggest threat thanks to the pistol. The rest should just use muskets. Help your allies pick them off. Another 2 groups of 2 wander into the fight from behind the rocks. They’re no threat. The only trick is watching your fire, since one of them should have an aura. There's no sense in losing upgrade points for no reason.
Once they’re dead, run up the road to the marked house. If you want, you can stop here and help the Union soldiers hold the gate and kill the 6-10 Confederates who flank it. You can also run past them and go to Dexter at the marked barn. It really doesn’t matter, since the Confederates are outnumbered anyway. If you want to stay, then stick by the fence and start shooting.
Switch to the musket before you talk to Dexter. He’ll have a new toy for you to use. You now have official permission to use the AR 55 against the Confederate line in the cornfield. Yes, it's as fun as you think it will be. Move to the east and rejoin the Union line. Take up a position behind the first firing line and just watch. When the Confederates make a push through the cornfield, just start firing. Spray and pray works just fine for this part of the battle. Your goal should be to cover as much of the field as you can. You should be able to practically coat half the field with one magazine. You also only need to save about 120 bullets for the next part, so have fun with this. The firing lines aren’t worthless though. They’ll actually handle a fair bit of the Confederate advance on their own. You just need to ensure that as many of them survive as possible, if you want the next step to be easy.
There should be 2 big waves of Confederates before they stop the attack. The Union then decides to advance.