Its not the barrel thickness inside the shroud that's the problem - its that if the same thread is used to locate the barrel into the breech, its the breech section that gets really thin...it'll manage just about, but the barrel wall at that point is mucho thin and just not very 'manly'.
The other potential issue with unknown conversions is whether the mag has been indexed accurately when its been opened up, but if you've used it and it shoots well still then its obviously OK