Kalashnikov was trying to build a replacement SMG for the Red Army. And he later scraped that Idea and built the AK-47 after the war. The main reason is that during the end of WWII. The long rifles and claibers of the world powers didn't work for street fighting. Which was becoming the norm. The 7.62X39 M43 Carridge was designed and made in 1943. Hence it's name. ANd the SKS Carbine was actualy first built to replace the VST-40 and M1891 Mosin Nagant rifle. That was to be chambered in 7.62X54R. The M43 cartridge was first built for a carbine type rifle. Something along the lines of the M1 Carbine. But it was later used in the SKS Carbine, which goes by he Soviet name as the Siminov 45 Carbine. Kalashnikov later took his sub machine gun deisgn and pllpied the same princeable as the STG44. And hence the AK-47 was born.
I've fired both the STG44 and a real soviet AK-47. Both have about the same recoil. But the AK-47 is a far better rifle than the STG44. The STG44 is simple next to the M1 Garand, SVT-40, and Gewehr 41. But when you have the AK-47 and the STG44 taken apart. You see the difference. If I was armed with a STG44. WOuld I feel under armed. No way. It's a great rifle for it's time. And it they were made with good German tooling. Like 1936 period tooling and metals. That rifle would be great. Also the 7.92X33mm cartridge is really cool. The DDR (East Germans) made STG44s until the early sixities. And when Uday Saddam and Qusay Saddam were killed in Iraq. They were killed holding STG44s. Uday loved that rifle. I've also seen some STG44s floating around in Taliban hands during my time in Afghanistan. They do work. And they work good. I just hated destorying those rifles. I cry sometimes when I think about the firearms that were burnt and crushed.