Thanks for the replys,
I am building a single shot, bolt action toy and set these requirements for the project...
it must use an m16 bolt and barrel extension (I have these)
it must use a 9mm/.357 barrel blank (I have this)
After taking measurements, I did not like the idea of opening up the bolt recess the 16 thousandths that the 9mm para case needed. That is when I thought of using the shortened .223 case (it fits the bolt) and opening it up to 9mm (it fits the barrel). This would have been a nice solution except that it would leave me with a round that could not be purchased over-the-counter if need be.
I think my solution at this point is to chamber for 380 auto. The case head is 4 thou smaller than the .223 (it fits the bolt and extractor nicely), it's a straight walled case (easier to chamber), it fits the barrel blank, it's readily available at the local ammo counter, and if it does not work I can still rechamber to something bigger.
Thanks again, dave