I lent my TC Hawken to a "friend."
He kept it two years, just wouldn't return it when I asked.
When I finally got it back, he hadn't cleaned it properly, the barrel was ruined.
I got a friend at a machine shop to drill it out.
I just bought a drill bit at a hardware store. It is about a .53 diameter bit.
My buddy welded the six inch bit onto 30 inches of 3/8 steel rod, making a 36 inch drill bit.
He fit the barrel, and the bit, into this giant green machine.
A little tube squirted machine oil into the barrel, while it was drilled out to the larger size. Took about 20 minutes.
I don't know what it would have cost, my buddy did it for free.
I had to polish it up quite a bit, but it made a nice little shotgun.
With number 12 shot it is hell on the carpenter bees.
You could make a larger gauge shotgun from a .50 barrel