First measure the bore diameter of your gun. That is the distance from one land to the one opposite it. The land is the " HIGH " portion of the rifling. It should be about .500". Use a caliper to do this.
If its a standard .50 caliber barrel, with a bore of .500, or .501, or, maybe even .502", You can use either. How accurate a give ball is depends on the patch thickness, and the Rate of Twist in your barrel. That is, how fast is the rifling spinning that ball?
While you are measuring bore diameter, measure the groove diameter. The depth of the groove often dictates the thickness of the patching material you use with a given ball. The patching has to fill that groove, tightly, to keep gas from cutting the patch, and even cutting the ball, which sends it off downrange who-know-where.
.490 balls tend to be easier to load, with a .015, .012, or .018" patch. With the .495" balls, they load a bit tighter, and many target shooters use them, or even .500" diameter balls to get the best accuracy from their guns. Sometimes, mallets are used to seat these large diameter balls.
How much accuracy you need depends on the use you are going to make of the rifle. If you are mostly shooting off-hand and hunting, the .490 ball is much easier to load in the field. If you are a target shooter, then consider the .495, and .500 diameter balls, along with teflon coated patches.
The best advice we can give you is the same advice we all received, and some of us followed: Buy some of each diameter, and try them both. Just be sure to measure the diameter of the balls you do buy, and shoot, to know exactly what you are shooting. Accuracy is all about consistency, and consistency requires uniformity in your components. Uniform weight, uniform thickness, uniform diameter.
Welcome to the sport, and to the forum. :thumbsup:
