Dave,
It's a Yes" and "No" kinda thing here. Yes, the dimensions are virtually interchangeable. Yes, 5.56mm ammo will chamber in a .223 Rem . . . usually. There are some very pronounced differences in the throat and leade areas between these two (which can jack-up pressures regardless of other factors), and the mil spec 5.56mm is loaded to higher pressures than SAAMI .223 ammo. Add to this the number of custom reamers/chambers in use that aren't entirely fish or fowl and there is a potential for trouble. Ditto for the case of using 5.56mm ammo in .223s in weaker actions than say, a bolt gun. A break top just isn't the place for pressure problems, and I'd strongly advise against using anything other than precisely what the manufacturer recommends. As I've said before, you're issued exactly two eyes and ten fingers. Balanced against that, cheap (or even free) ammo is just no bargain.
If you want to use that nice, inexpensive military brass that BHO is no longer trying to keep from us evil reloaders, that's just fine!
Hope this helps.
Kevin Thomas
Berger Bullets
