Dennis, how ya' doin'? Lots to consider but, you seem to have a pretty good handle on it all! I'll address headspace. You won't be duplicating the chamber dimensions of your 22-250 with a fire formed case. You'll only be duplicating the dimension of the case. Remember, the case expands on ignition then rings back to allow easy extraction! It's only a few thou. but critical. You could get the headspace so short that your full length die won't be able to move the shoulder back far enough to chamber the cartridge. I once had a gunsmith/blacksmith do this to me!! The specifications on chamber reamers run all over the place within the "saami" spec. I think that to be safe, a 22-250 go-gauge would be in order, especially on your first build. Jim @ NSS gave me a good tip on head spacing my .223. Set the headspace to the go-gauge then apply a piece of scotch tape to the "head" end of the gauge. Trim the excess tape and chamber the go-gauge. The bolt should NOT close. Tighten just a tad, if necessary, 'til the go-gauge goes and the taped go- gauge won't. The scotch tape will measure .0015" to .0018" Send the no-go back to Mid way and exchange for a 22-250 go-gauge! Works for me!! Good luck and keep us posted on build. Almost forgot, nothing special to do with .223/ mil spec brass. just keep it seperated as case capacity "due to thickness" varies between the two. A good shootin' .223 rem. load COULD be too hot for the military stuff. This isn't always the case ,Pardon the pun, but better safe than sorry!!