Dave,
1 lead smith, 3 labors=4 men x 10 hours= 75 to 80 barrels (WELDED not bored and rifled) starting with a skelp. Reference "Quest for the Indian Trade Gun".
I believe with starting the fire and the coal coking up, once we got into the rhythm ( of Galley slaves) the work wasn't that bad. The work including flattening the wagon rim , cupping the skelp, then welding the skelp together 12 hours give or take. I do believe it could be done in a shorter time with more practice. My guess with practice 4-5 hours with 4 guys.
All this welding just to get a deep hole through a bar of iron, comparing modern deep hole drilling, I have been told usually 30 mins to drill a deep hole in 48" barrel.