Why don't you just buy Goex brand BP, and stop shopping these foreign brands? Elephant powders have received mixed reviews, at best, since they hit the market, and seem to be going down hill. The Swiss powder is really expensive. Its good powder, but it costs almost twice what you will pay for Goex.
You have a computer, so why not Google Goex, find the Distributor for their powder for Negraska, and call him on his 800 line to find out where you can buy black Powder close to where you live. If there is no one close, they will arrange to ship it to you directly, and at a cost you can afford. Just don't be buying it at one can at a time, since there is a hasmat fee that has to be paid, which is about $20.00 per shipment. If you have to , find a couple of friends to place an order with you. There are links on this sight to dealers who ship BP all over the country, in small lot orders at a savings to you.
Youhaven't told us what thickness you were using, or what the new patches thickness is. No one can tell you if you are going to have problems with a .490 ball until you tell us how thick your patching is, and what is the real exact dimension of those cast balls. They may be .003" plus or minus that nominal .490, depending on the quality of the lead you use, the individual mold, Your casting techniques, etc. The .490 ball is a good size to use in most .50 cal. rifles, with a .015" patch that is well lubed. There are many recipes for lube posted here. If you read the forum daily, you will find the recipes repeated often, as new shooters ask about what to use. Try to buy the same components for your gun each time you purchase new supplies. Consistency is needed to get any accuracy.
As to the question as to why your groups open up at 100 yds more than you like, there can be dozens of reasons. Have you asked someone else who is a good shooter to shoot your gun? That is the quickest way to find out its not the gun, or load, but the shooter pulling the trigger. We can help you on that. If the gun does not shoot well for anyone, at 100 yds, then the first thing we have you do is look at the spent patches on the ground in front of the shooting bench, and tell us, or take a picture and publish it here so we know what it looks like. If your patch is too thick, you can stretch the fabric, or cut or tear it, and that will allow gas to cut the RB, so that between 50 and 100 yds, the ball begins to wander off somewhere you didn't intend.
Buy Dutch Shoultz's BlackPowderRifleAccuracy system, at
[url] www.blackpowderrifleaccuracy.com[/url]. Its only $15.00, and you will learn a step by step method of diagnosing your gun and load so you can create an accurate load for your gun. Many of us have used his system, and we all like it. It works. And it will save you a lot of time and frustration.