As they say, a picture is worth a thousand words…
Here are two cases I found at the range last week.
On the left is a properly fired 7.62x54R case. On the right is a 7.62x54R case that was fired in a rifle which was decidedly NOT chambered in 7.62x54R! It should go without saying that this is dangerous and can end badly. If you feel resistance when chambering a cartridge, do yourself a favor and check out the round before you try to fire it. If you don’t believe me, watch as this guy – who clearly had ZERO experience with the AR-15 platform – manages to blow up his rifle due largely to his own incompetence. Now, that case didn’t involve an improper caliber, to my knowledge, but it still serves to drive home the point that forcing a round into the chamber is likely to have negative results.
Update with another photo. I was thinking that it was .303 due to the new fireformed rounded shoulder, but I had to dig out some brass to check.