Jim B: Sorry, I haven't heard of that firm but Stan and BigBill have given you good information. Yes, you can load the 303 Brit with the 123-5 gn 7.62x39 bullets for light loads - years ago the 303 Brit even had a loading with that weight bullet.
Or, you can e-mail MCA Sports in Anchorage, Alaska and see if they will/can make up a 7.62x39mm insert for the 303. Much cheaper than having a rifle rebarrelled. I have some of their inserts for my 30-06 and was just playing with 7.62x25mm ammo in that rifle - shoots like a hoot and works like a charm, it's a blast. HTH. Mikey.