This is a size comparison between a Chinese surplus .311-caliber 7.62x39mm (designed in 1944 for the RPD and then the SKS) next to a modern production Wolf steel-case 5.45x39mm (the round designed in 1974 to replace the 7.62x39mm.) Think of the 7.62x39mm as a smaller .308 and the 5.45x39mm as a "hot .22". The Russians designed the 5.45 to have a thinner, longer bullet to travel fast and yaw and tumble when hitting a target to increase the likelihood of hitting a vital organ or vein. They designed the 5.45 AK-74 to phase out the 7.62x39mm AKM and jump on the "hot .22" bandwagon instead of continuing to use slower, fatter bullets.
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<span class="int-own-work" lang="en">Own work</span>