I personally would fire 5.56 in a .223 chamber any day. This is my belief. I have looked at the facts and sought advice from people I trust. I know why people believe it shouldn't been done but I disagree with those points as I believe they are insignificant and unlikely. As I say I have looked into this & would feel fine doing it, BUT it is up to you to make your own call on that.
I think a lot of the hype is fuelled by the attorneys for the manufacturers getting it put on there so they can say "Oh his gun blew up and you're suing us? What ammo was he using? 5.56 you say? Our manual specifically states not to. Sorry, no case." to cater to the lowest denominator. European ammo is loaded hotter than American ammo (or can be by CIP spec vs. SAAMI spec). Once again I believe this is due to the American sue happy attitude (not saying all Americans are that way but it is only America that is that way) It’s the whole "Well the gun says 8×57 so it should fire any 8×57. XYZ Ammo Co. seeing as he was using your ammo at the time pay him eleventy billion dollars" so American ammo is lightly loaded to compensate for that while in most other countries its “You fired modern ammo in a WWII gun that hasn’t been fired or maintained in 40 years w/o getting it checked first? Case dismissed." After granddads 8mm Mauser has blown up in your face.
SAAMI max pressure for .223 is 55k PSI. CIP max pressure for .223 is 62k PSI. Same as NATO's max for 5.56. Euro ammo is just as common as Yank here and people have been firing it in Yank rifles for long enough with no ill effects from the pressure.
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