Starting in the 1960s the militaries of the world began adopting some sort of automatic grenade launcher:
https://en.wikipedia.org/wiki/Automatic_grenade_launcher
Ones of these in particular reminded me of a WW2 weapon:
https://en.wikipedia.org/wiki/AGS-30
https://en.wikipedia.org/wiki/MK_108_cannon
The Luftwaffe weapon above was a heavier, much higher velocity version of the Soviet 30mm automatic grenade launcher, which seems like it would be pretty easy to develop a ground version for.
So what if in WW2 someone, probably but not necessarily exclusively the Germans, adopted an automatic grenade launcher as a standard weapon for infantry units and vehicles with performance like the Soviet AGS-30? Or even an adapted MG FF with 20mm 'mine shells'? Both the MK108 and MG FF were made of stamped metal parts and were ridiculously easy to make with very powerful ammo and would have been even cheaper and lighter with lower velocity ammo.
MG FF in AA ground configuration for reference:
https://axis-militaria.com/product/...sory-part-20mm-mgff-aa-ground-variant-v-rare/
Of course the Soviets also experimented a 40mm weapon that they dropped in the 1930s:
http://sovietguns.blogspot.com/2013/12/taubins-grenade-launcher.html
What sort of impact would these weapons have had on infantry combat?
They have far greater range than a light mortar, better direct fire accuracy, and can bring down a high volume of fire on charging infantry or a fixed position that a .50 caliber MG would envy.