I can answer those questions. The inserts are legal to own and use. It is/was common for people to use 22lr inserts in shotguns to quietly dispose of garden pests and the like. These inserts are made in some rifle calibers all the way up to 45-70 for 'insert' use in shotguns and pistol calibers (38/357, maybe some 32s, 44 and 45 acp or Colt) and for use in shotguns and same bore size rifles.
The inserts are currently made. You can sometimes obtain inserts in 32 aCP, 30 Luger and 30 Mauser for 303 Brits, 308s 30-06s, 7.62x54R from Sportsman's Guide. Reduced caliber shotgun inserts are also available to larger bore shotguns as training aids or for light field loads. I think Gaugemate (?) also makes them.
They are fun to use and shoot but I have not yet gotten much accuracy from any of those I have used. I think there is a pretty long 'jump' from chamber to rifling though and that doesn't seem to help with accuracy. They are fun to use though and reloading may be the ticket to accuracy with these. HTH and good luck.
