Its just clipped into the lips of the mag before being fully inserted .

Fully inserted . when making its a case of trial and error with getting the angle right so it sits in the mag correctly and filing a groove for the round to sit in .

A view from the front , if you drill a small hole at the front above the top of the mag you can insert a pointed object to extract it from the mag

A dummy round sitting in the groove .

Aview of the breech with dummy round presented at the correct height and angle to slide into the chamber

Hope this shows how easy these are to make .
Dave