Moloch, once you start to shoot your new rifle, you will look back and laugh for being worried about shooting it!
I don't know what you've been reading about, but it's certainly not something that happens everyday.
These guns are safe to shoot at least a maximum of 100 grains of ffg by volume. fffg burns cleaner and faster, so if using it, just reduce the volume by 10%. I have the a Pedersoli with the thinnest .50 caliber barrel (13/16th's of an inch), and I have shot 90 grains of Pyrodex P out of it with no problem at all. And Pyrodex P has more pressure than BP. You shouldn't worry so much about it.
A person who doesn't ram and seat their projectile all the way down might be able to put a slight bulge in the barrel, but exploding a barrel is pretty rare. People have tried to do it and they can't, even using 2 balls and double the powder.
I started my son shooting a .50 caliber sidelock at about 9 years old. We loaded 30 grains for shooting at 25 yards, and 40 grains for 50 yards. But usually the common starting load is about 45 - 50 grains. The less powder that you load, the less fouling and sometimes even the better accuracy. Because it has a longer 35 inch barrel, it will develop good velocity, and maybe even a little more fouling down in the breech because not enough of the patch lubrication will always reach down there.
When you want to shoot for more power, you increase the load by 5 grains each time while testing the accuracy. When the accuracy starts to get worse, you swab the barrel and dry patch it, and try it again for accuracy, and reduce the charge accordingly.
Some guns like more powder, some guns like less. It will take a little time to find the best loads.
So don't worry so much about the barrel exploding. The .50 caliber barrel develops less pressure than the smaller .45 bore because it's bigger.
60 - 65 grains is a moderate and very safe load that most people are very happy shooting .50 PRB's with, and I often shoot with only 50 grains of Pyrodex at 50 yards.