I have seen a Model 39 with the barrel extended about 10 mm with a machined bushing. The end of the barrel must have been turned down and this bushing shrunk fit to the barrel. The id of the extended part of the bushing was greater than the id of the barrel so the bullet would not actually contact the bushing.
It would be a fairly economical mod.
Gunstock
"...$300-$400..." More than the pistol is worth. If you want a 39 and you don't have the prohibited permit, you'd be better off buying something else.