Acer-m14
20×102mm Vulcan
guess before scrolling down to link on what happened ..
Firearm Safety: Always Double Check Your Ammunition
How though? It's ridiculously easy to tell the difference between 300 and 223. And then are you not looking at your mags before you take them to the range/load them? It's very easy to tell the difference between 44 special/44 mag, 38 special/ 357 mag, 9x18/9x19 etc. with the naked eye if you took just the casings and jumbled them up. I can't imagine how you would mix up 300 and 223 with the actual projectiles loaded into them. IDK.
Do you have a 300bo?
It is very easy to do. Rounds mix easily.
I have done it. Only thing that saved me is i load the BO so it wont close in a 556 gun.
Again, it is easier than you think.
Could be as simple as thinking you switched uppers, trying to stick some larger cal in and WHAM! Big bullet, small chamber...