Anyone know where I can get hold of some once fired brass? New would be nice too, but I would expect it to be much more dificult

Have you tried .221 Fireball brass?

No. But I was thinking that the rim diameter was larger on almost everything except this case. I was looking to stay with the 0.305-0.310 rim/body diameter. I may end up looking into the larger based cases, but every thou adds complexity to magazine considerations.