Given that everyting is the same as a .223/5.56 gun, what happens if you accidently run a .223 mag in your .243 and vice versa?
.223 will go bang safely, but will be terribly inaccurate.
6x45 in a .223 gun won't close battery. Pill will jam at the throat.
100% of my 6x45 brass is nickel to mitigate oops afield.
SINCE THAT TIME...
I did witness a similar problem...
A good friend and very competent GunSmith/FFL DID ACTUALLY
chamber and fire a .300 Blackout round in a 6x45 barrel... same scenario would be possible if the gun was a .223 How could this possibly happen???
The bullet set back into the cartridge case as the BCG closed battery. I assume insufficient crimp on the round.
No one got hurt and believe it or not... the .308 bullet DID leave the .243 barrel... however that barrel was irreparably damaged.
Lesson learned by me...
Check, crosscheck and recheck everything... every single time.
If you are ever in doubt... STOP!!!
OR... if you do some of the really really weird monster stuff I do... wrap the gun in a moving blanket, ratchet strap it to an oak tree/bench and get yourself a long string. (i.e. back when I was learning to play with 730 gr. .458 bullets I did just this).