Welcome to the forum.
Your .577 caliber Enfield was a very popular gun with the Confederates of the South. They imported thousands of them from England during the American Civil war.
The North also imported them, mainly to keep them out of the hands of the Confederates.
I assume you are asking about shooting lead Minie' balls, the original projectile that these guns were designed to shoot?
These large hollow based slugs were designed to be slightly smaller than bore size to allow easy loading even in a fouled bore.
On firing, the powder gas pressure expands the hollow skirt causing the outside of the bullet to engage the rifling grooves as it accelerates down the bore.
During the American Civil War, accuracy in the field was not terribly important for the average soldier but the rifling did give a greatly improved ability to hit the enemy at much greater distances than the formerly used round ball in a smooth bored musket. Enough history for now.
The thing we've found about these hollow based bullets is their design limits the amount of gunpowder that can be used if accuracy is the goal.
The typical powder charge was about 60 grains of rifle (2Fg) powder. Synthetic black powders like Pyrodex RS and Seven7seven 2Fg powder will work in your gun.
The nipple will be the large Musket Cap size. The Musket caps are much larger than the #11 percussion caps used on other percussion guns.
If you can't find any Musket caps, places like Track of the Wolf do make replacement nipples that have a cone size that will allow using the smaller #11 nipple.
I believe your Enfield nipple uses the nipple with a 5/16-18 thread (like the originals and the Parker-Hale Enfields).
Charges up to 70 grains will work in some guns.
Charges larger than this will usually cause the hollow base to expand or even blow apart as the bullet leaves the muzzle. That can totally ruin the accuracy of the shot.
Another thing that has been noted is the bullet must not be over .003 smaller than the bore size of the gun to be accurate.
The bore size in your Enfield is supposed to be .577" but variations in manufacturing can make your guns bore slightly larger or smaller than that.
That .577 bore size makes the desired Minie' bullet diameter .574-.575.
Lyman makes a bullet mold specifically designed for the .577 bore Enfield. It is a 566 grain slug making it about the same as a 12 guage ball in weight. The bullet mold number is Lyamn # 575213PH. (The 575 represents the diameter which is .002 under the .577 bore size).
The Minie' bullet is loaded without a patch and with a light coating of grease in the grooves.
More than a few of us have found that shooting a .570 diameter lead ball wrapped in a lubricated cloth patch gives good accuracy.
It's lighter 278 grain weight produces much lighter recoil and the balls are sometimes easier to find than the Minie bullets.
Have fun with your gun.