Sounds like a nice barrel. If you have calipers - dial of not, a good measurement can be obtained on a bore that size.

A 5 bore sized ball should weigh 1,400gr. in pure lead. I assume from past writings, that around 1,400gr. to 1,500gr. was the size most used.

As the great 4's were used mostly at spitting distances, they wanted a ball that required little or no resistance to seat on the powder. We know this doesn't produce good accuracy, but at very close range, speed of loading was more important than accuracy. A lightly patched smoothbore will shoot right along with rifles to about 25 or 30 yards and produce 1 hole groups - good enough for the huge beasts od chase.