A smaller caliber shot in a larger caliber is more forgiving. I state this as I once fired a 357sig round in a 40s&w. The recoil felt weird, but no harm was done. It cannot over pressure due to having a larger bore to pass extra pressure by. Having 6.5gren, 6.8spc, 5.56nato and 300bo in AR-15 so I can check for all misshap in cartridge interchange by bolt closure. I'll remove firing pin before testing. It all comes down to AWARENESS! Be safe! Glad I wasn't worse!I had a local youngster bring me a 6.8 SPC AR night hog hunting rifle with thermal scope.
He said its hanging up after firing and won't cycle the next round. Failure to feed.
When he arrived and handed me the rifle and mag... I took a quick look and asked if this was the exact mag and ammo that was giving trouble. He said, yes sir.
I handed it back to him and said... Here is your problem...5.56 ammo was loaded in the mag in the dark of night and run thru the 6.8 rifle.
Dangerous situation.