Hi JOhn.

I will check in on who does M03 spare barrels besides Mauser. I am sure there is a few who can do it...and do it on regular basis.

The Mauser 66 was an expensive rifle to produce and still remain a good platform for just about any caliber up to COL 3,75" long. They have been made in .416Rigby, .378WEa etc. It still has a following crowd and that's why we see german gunmakers has found a small special niche production to make sparebarrels. At some point it was a rifle ahead of it´s time.

The Mauser M03 is a rifle that is still being produced alone mit-habengut-und-alles.(everything)
Any good gunsmith ought to be able to make a spare barrel for the Mauser M03 from a raw rifled tube.