https://www.youtube.com/watch?v=-2oDJDgEGGU&t=587s
Interesting video about the powder charges used on US battleships, specifically the 16" 50-caliber (20,320mm barrel) guns. The specification I found said 20m "bore length", and 20.7m barrel length to breech face, neither of which works with a 50-caliber, 16" gun, or the shorter 45-caliber gun. Unless they actually jam the shell four-meters into the rifling, they're not going to fit 2.7m of powder charge behind it. I couldn't find a definitive length for the shells, but scaling a photo it looks to be roughly four times the diameter, or about 1.6m long, plus the 2.7m of powder bags, making the "cartridge" 4.3m long.
4,620,000gn charges were standard, but dropped to 4,585,000gn charges post-WW2. The 297kg charge column is 450mm diameter by 2700mm long!
During WW2 the barrels had a life of 290rds, the reduced charge extended it to 350rds. They eventually got it up to around 1500rds barrel life. The barrels are 25-caliber-twist, or 10.16m per revolution, equivalent to a 5.6"-twist .223Rem (8"-twist .223 is 35.7-caliber twist). 96-groove rifling, .150" deep.
Particularly interesting is the 5fps variation in velocity at 2425fps with the 1200kg AP shell (2615fps with the lighter 860kg HE shell) during WW2, just astonishing. And also that the powder deteriorated over 50+ years (it was originally manufactured in the 1930's), although I'm not sure he's explained it quite correctly. After more than 50 years, the powder deterioration increased the average ES from a WW2 ES of 5fps, up to a 32fps average ES, which becomes a problem in long-range accuracy. He seems to be explaining it purely as a loss in velocity, but it's actually a loss of accuracy due the increased variation in velocity. A difference of 32fps in muzzle-velocity at 1000m isn't much at all, but by 38km it's getting significant enough to risk unnecessary collateral damage.
Also interesting that they "hand-loaded" the powder bags to ensure the weights were correct due to variations in the sizes of the "grains" that were stacked into the bags. They basically filled the bag with a low charge, then "trickled" it up to weight by laying additional grains on top.
The only ballistic calculators I've found that allow ranges of 38km or more, all ignore air drag, and thus, BC entirely (BC is a drag "factor" so is irrelevant in vacuum). In a vacuum, a ballistic (unguided and unpowered) projectile (a bullet is a specific type of projectile) follows a parabolic trajectory, in air it follows a ballistic trajectory due to drag bleeding off the velocity. In space, the 16" gun would send its 1200kg shell out to 55.133km to 55.372km (240m spread due to 5fps variation in MV) fired at 45-degrees, on earth it is launched at around 40-degrees for a maximum range of 38km. Launched 16fps slower or faster would push it to 54.52km to 55.991km, a 1470m spread over the target. These numbers would be reduced due to drag (in the video he says it's roughly 500yds short/long), as with the maximum range, but I would expect drag would also decrease the accuracy anyway.