I'll send you a picture when I get home, but when you cut off the forebarrel, you'll see what I mean. It's mostly 3/8" inner diameter inside the barrel, but there's some occluding inside the sight portion. Carefully drill that out with a 3/8" drill bit to even it up. Do you want me to whip up a forebarrel for you like what I showed above? Get a 3/8" brass tube at your end and you'd just be able to slip the assembly on and do up the Tomtit part and muzzle with a minimum of fuss...
--Jonah
--Jonah