Some of the responses here on this thread make me think -- forgive me -- that some people here have been watching too many war movies, and are short on military experience. Folks, there is a reason that no army in the world is issuing full power battle rifles, let along single shot breech loaders. I agree that a skilled rifleman can do some pretty amazing things with a Sharps. But think about the missions you would likely have to carry out in wartime. Do you want to be walking point down a road, in front of a platoon of fellow soldiers carrying a single shot rifle when you find you've just walked into an ambush? Do you want to have to try to lay down cover fire with one for your squad mates doing 3 to 5 second rushes up a hill to take an enemy position? More importantly, do you want them trying to lay down cover with such weapons for you when it's your turn to get up and rush forward? Do you want to have a company of enemy soldiers charging your foxhole with only a single shot to fire back at them?
It's called fire superiority folks, and soldiers want it. And fire superiority consists of accuracy and lethality, but it also, in modern warfare, consists of volume. Let's not forget that the vast majority of killing in warfare is done with artillery and support weapons. Literally tens of thousands of rounds are fired just to keep the enemies' heads down for every one that kills and enemy soldier. It's not enough to have the most accurate or hardest hitting rifle. You also need to have the one that lets you carry enough ammo to achieve fire superiority.
Now one might say "why worry about the rifle at all, if so little actual killing is done with it?" The answer to that is that because the rifle is a soldier's personal weapon, it matters a great deal to each soldier. If you're clearing a building, and you step round the corner and find yourself face to face with an enemy soldier, you want a weapon that will put him down before he can do it to you. If you are in Afghanistan, and Taliban fighters open up on you from six or seven hundred yards away, you want a rifle with the range to return fire effectively (which the 5.56mm won't). If you are in foxhole and the enemy is threatening to overrun your position, you want a rifle that will allow you to lay down enough fire to make them break off the attack.
No one weapon will be ideal for every mission a soldier has to undertake. The assault rifle is now standard worldwide because it comes the closest to meeting the requirements for the widest possible number of scenarios. Want more range and hitting power, the tradeoff is that you can't carry as much ammo, and controllable full auto may go out the window. Want to carry several hundred rounds, the tradeoff is having to go to a smaller caliber like the 5.56mm, which may lack the range and/or penetration for some jobs, and which is heavily dependent on bullets yawing and fragmenting for its lethality.
The best assault rifle, IMHO, would be in a caliber like the 6.5mm Grendel, or a ballistically similar caliber, because it's reasonably small and light, meaning you can carry an adequate number of rounds, and recoil is not such as to preclude selective fire, and it's got the range to do about everything the 7.62mm NATO can. A weapon like the AR, with it's excellent ergonomics, would be a great platform for such a cartridge as well. Pity we don't have something like that, or any realistic possibility of getting it in the near future.
But as for going into battle today with Garands or Sharps rifles... Folks, if those were still the best weapons for the job, soldiers would still be using them.
Not for ordinary grunt-work.
