TBF those are WWII lend-lease guns from the US which not many are familiar with. Retraining troops to use guns that are not in production in Russia is a pretty dumb idea. The only useful ones I think are the Maxim machine guns, and even then they aren't exactly great.Yeah didn't they found like a massive weapon and ammo cache in Soledar salt mines enough to arm a million man army.
Not to mention some of them might have been sabotaged. So the risks aren't exactly worth it.