Sub caliber insert,.. Numrich made the famous '03, '03A3 unit back in the 60s. They also made one for the SMLE. It consists of a tube barrel insert, with nut at muzzle, and a complete bolt with .22 cal. bolt head. I have one stashed away somewhere.

I now see n64atlas' postings... I didn't even see page 2 &3 until after I posted...sorry! My .22 conversion unit is just like his.