The chamber is larger than the bore. It has to be in order to even get a bullet in there. The bullet is supposed to be a bit larger than the bore of the barrel, and therefore the chamber behind the bore has to be bigger still. That is so the bullet gets a good seal and engages the rifling. Sharps are more accurate than muzzle loaders, which need undersized bullets, and seal those using patches of cloth, but not so tight that you cannot ram it down the barrel. That makes for a moderate fit and engagement of the rifling at best. That is unless you hammer a larger bullet down the barrel with a mallet and ramrod strong enough to take the punhishment. I know that some old time hunters did just that, but they had time between shots to do that. Combat rifles like the Sharps can get you killed if you take that long to reload.
It does indeed have a sleeve inside the chamber. It is a metal bushing that you can pull out of the rifle, though it will NOT come out the rifle the whole way. You would need to take the barrel off the rifle if you want to get it out or replace it. It is called the Gas Check.
It is for making a tight gas seal when the gun goes off, being pushed against the breech block. When you get home at night you are supposed to clean that gas check a lot more than any other part of the rifle. If it doesn't get cleaned, it will lock up and you might not be able to get the breech block to open because the sleeve is pushed against it. It won't budge due to the dirt/residue build up and force you to scrape the face of the breech block against it to open it. Clean it, and oil it!
There are special tools you can buy to be able to move the gas check if it really is stuck. The one from Pedersoli is supposed to be better than the one that Dixie Gun Works sells. It looks like a handle with a split tube on the end. You put that in the back, tighten down then handle, which spreads the two halves of the rubber tube apart wedging it in and giving you a good grip on the gas check. The metal rod sticking out the side is to hold the head steady while you do the torquing down with the handle.
I might skip buying a tool and just cut a .50 caliber (half inch) wood dowel rod in half on a bandsaw at work. I will use a large, flat tip screwdriver to wedge the two halves apart
If you are worried about the size of the powder charge being too much, I don't think anybody in their right mind would build a rifle that can be over charged. Also, the cartidge guns get up to chambering brass that is .45 caliber, and have powder loads of 70 grains, 90 grains, and even 120 grains of black powder. The 45-70 round, 45-90, and 45-120.
You may also want to buy bullets that are not hollow base bullets, and better still get some longer bullets. Dixie Gun Works sells bullets specifically made for the Sharps.
The last part of the bullet is a smaller diameter, so you can glue or tie a paper cartridge to it.