I have a number of 5.56mm (.223) ex military cases that I want to size to .222. I have tried one and it came out ok, but I recall reading something about mil brass and reloading but cant remember what it was. Can anyone tell me if they have tried doiing this and is it OK?
Cheers

Check two things carefully: the case volume in compaison to "real" .222's and the neck thickness.

Military cases are a bit thicker, so they usually have less volume...so your .222 case fromed from .223's may have significantly less volume. Smaller internal volume will raise the pressure of listed loads...would take that into account.

Also becasue you are forming .222 case necks from the thicker areas of the .223 case, may not allow enough bullet release "slack" when loaded as .222's. Your average factory loaded .222 will measure about .253" over the case neck with seated bullet....if the case necks are thciker than this, then the measurement over a seated bullet will be larger. IF large enough that the bullet can't release cleanly when fired, pressure tends to sky-rocket.

May have to neck ream/neck turn to get the case necks as thin as issued .222 cases...a lot depends on the chamber in which they are shot. Some "sloppy" .222 chambers will accept and work fine with necks a little thicker than standard, but most of them are pretty well cut and don't take well to over-thick necks,

Ditto what ribbonstone says -

Main difference is the wall thickness of the brass. It's a doable project, as long as you keep in mind the diminished case capacity and neck ream/neck turn to assure no tight necks in the chamber.

Pass over this if you like, just an illustrative story:

Back in the days before commercal .30Maurser/.30Tok cases would form them from .223's and .222's. Understood the importance of neck reaming, and reamed the little stumpy cases fit the Tok's chamber correctly with enough bullet clearance.

Later on, one of the CZ52's pass theough my hands and I load up some of that left over converted brass. About the fourth shot or so, noticed that the ejected cases looked just like 9mm's. Turns out at each pull of the trigger, the CZ was sending both the bullet and the case neck down the barrel....much tighter chamber than that Vietnam take-back Tok, no room for bullet release, so the only two choices were to blow up the gun or take the case neck along for the ride down the barrel....kind of glad it took that last course and not the first.

Less on learned: Don't rely on book-numbers, each chamber is different and for safety better to take that into account than rely on what the spec. sheet has prineted as "right".