Reaming and rifling a wrought iron barrel.

[sportster73hp]

Why stop at 38? A .400 or 40 caliber is just a little bigger. Use a reamer or hone it to the next size, you have already gone this far.

 

[Rifleman1776]

For PRB, you want a slower twist. 1:66-72 is common. 1:48 works, but is a compromise for using bullets in the same gun.

Big disagreement here. 1:48" twist is great for round balls and is a compromise for nothing.

 

[Rifleman1776]

I intend to drill it out to 9mm, ream to .358-.360", and single-point rifle it by hand the old fashioned way. I will also taper and flare it because it is ridiculously heavy.

If it can be done safely, a larger bore will lighten that "ridiculously heavy" barrel a bunch.