I used the 4 shot magazine for the Savage 340 rifle. It fits inside the shell nicely, a wedge of wood underneath it and some shims on each side meant I could affix it in the correct place for the bolt to pick up the next round and I then filled the case with Epoxy for a permanent fix. I used a No.4 Extractor ground down to make a small feed ramp for the cartridges so that the pointy bit enters the chamber.
DO NOTE - if you use epoxy, for goodness sake don't forget the stick tape over the hole in the bottom of the 303 magazine or it gets very messy very quickly.