To answer your ‘why’ [the two are not interchangeable], the bullets/projectiles may be of equal diameter, but the shell cases are not. They differ in shape all together. So the case of one will not fit into the breach of the other. Even if the length was the only difference, it still would not work because the neck of the case needs to fit snugly into the breach while the back/rim needs to be held by the bolt and struck by the firing pin. So an exact fit is crucial. The left two rounds are 7.62 x 51 and 7.62 x 39.
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