9mm/38 bore reamer size?

[Riven]

I was thinking of making a 12ga to 38spl adapter as a see if i can project.
Going to cut down an old shot out enfield barrel but im not sure of what size i should be reaming the bore to.

 

[Blastattack]

Is this adapter rifled or smooth bore?

 

[Riven]

Was going to rifle it hopefully.
Amazon has buttons for a fair price so i figured why not give it a try.
38 seams to be a nice safe low power option for my first try.

 

[Blastattack]

Was going to rifle it hopefully.
Amazon has buttons for a fair price so i figured why not give it a try.
38 seams to be a nice safe low power option for my first try.

SAAMI specs for all cartridge dimensions are on the SAAMI website.