http://www.luckygunner.com/labs/5-56-vs-223/
It depends on which way you consider there is a safety issue with .223 /5.56
Again, Mossberg reckon its (at least partly) ok...
The MVP series of firearms are designed to operate with ammunition of the type
and caliber inscribed on the barrel.
NOTE:
Firearms inscribed 5.56mm NATO can also fire 223 REM cartridges.
Firearms inscribed 7.62mm NATO can also fire 308 WIN cartridges.
Now if you have a firearm inscribed .223, and you use 5.56....???
It's a can of worms.
Just because it says .223 Remington doesn't gurantee it's chambered .223 Rem. It could be chambered 5.56 and marked .223 for various reasons.
Take the Mini-14 for one example.
Comparing my brand new stainless Mini (1982) to my cousins early one made in 1976. Chamber cast of his measured out same as .223 Rem and mine measured out 5.56. Both rifles were marked .223 Rem. Only thing I can think is Ol Sturm Ruger wanted a bigger safety margin on account of surplus ammo or some more reliability for their rifle that was trying to compete with other self loaders for market share.
Take exports/imports for example too.
Importing rifles marked 5.56 to Canada and a bunch of other countries is forbidden by their laws. The difference between 5.56 and .223 chambers can fall withing normal manufacture variance. Somehow I doubt anyone checks chamber dimensions before signing off the release documents in the customs warehouses. Good thing to as it makes no real difference in new guns anyway.
Where I set up and take notice is milsurp ammo.
We knew milsurp ammo was a little hotter than SAAMI ammo but back then there was no green tip or 77 grain stuff.
If it was milsurp it was all 55 grain. It shot hotter, we could hear it and measure the velocities but the primers never flattened or cratered so if we found it cheap we used it for fun or carried it in the tractor gun and saved the good hand loads with premium bullets for real shooting and busting wood chucks.
Then the green machine ( Army not RCBS ) came out with M855 green tip and M16A2 and things changed.
The bullet of the M855 round had a different ogive, it was loaded even hotter than the M193 stuff was and the chamber drawings were changed too. No one I know wanted to stuff that stuff in their Savage 340 unless they chamber was throated realy long. And then it didn't shoot worth a dang. It needed a good 10 twist or better yet a 9 twist to group half way decent as far as Ball ammo goes. The 55 grain M193 stuff shot better but was ho hum to begin with anyway.
Between then and now we learned that 9 and 10 twist gave the green tip stuff its best accuracy ( as far as ball ammo goes) but the military went with a 7 twist to stabilize the new tracer which was incredibly long for its weight.
So the moral of the story is if the manufacturer tell us it's safe to shoot either or in my gun I take it as gospel and don't loose a wink of sleep over it.
If it's marked .223 I check it out anyway to see if it's chambered .223 or 5.56 because it's not likely to kill me but I want to know which way to go for better accuracy. Than Ball ammo still sucks in the accuracy department compared to premium fodder.
These other fellas over at lucky gunner labs did a bunch of testing using real science and it was pretty eye opening.
http://www.luckygunner.com/labs/5-56-vs-223/