I know an outfit that makes rifled choke tubes in 12 and 20 ga. They only have buttons in those gauges. I buy the rifled blanks and have them turned and threaded to fit my shotguns. The rifled blanks are $20 each and it costs me $75 each to have them threaded. They make a pretty good paradox gun with Brenneke conventional slugs.

If someone can identify a source of a 16ga rifling button, this shop could turn up a bunch of blanks, then rifle them by pushing the button through the blank.