I'm working up a load for IHMSA production class using cast bullets from a RCBS 140 Sil mold. The fire forming loads show potential. Using 223 brass I have about .050 to .060 more chamber than I have case neck when comparing the fired case with a chamber cast. When the bullet is seated against the rifling, the front driving band is not quite in the neck which leaves the bullet supported only by the rear driving band and the gas check. I'm considering forming cases from 222 mag and trimming the necks to .010 shorther than my chamber. Has anyone else tried this and is it worth the extra $10/100 and effort needed to trim and possible reaming/turning the necks?

That all depends what you think your time is worth. Personally I do not go to such great levels as .223 brass forms quite nicely and performs very well in "fireforming" mode.

I recall a few guys going this route a few years back. It helped correct a sloppy chamber some what. As to the brass I have heard "+" and "-" about the 222 mag brass. I would say give it a shot (batch of 100) and see if it helps with your cast bullets. I would recomend turning the necks rather than reaming, as neck turning will have "more" benifit for the time invested in case prep.
Jeff