The modern battlefield has a diverse variety of crew-served weapons, from machine guns to grenade launchers. Its the latter category, exemplified by the AGS-17 and MK-19, that have a lineage going back decades, including to a weapon of a different class but many similarities.
The Rheinmetall MK-108 was a 30mm German aircraft cannon that was a marvel of compact size and design and not unlike those modern crew-served grenade launchers of today.
The automatic aircraft cannon was created as a cheap, easy to manufacture system that would inflict the most damage with minimal number of shots. The MK-108 had 15 types of rounds developed for it, but most commonly fired 30 x 90mm 'Mine Shell,' essentially a high explosive fragmentary round, and an incendiary shell. The direct-blowback MK-108 had at a rate of fire of 660 rounds per minute.
Just over 3.5 feet in length, the MK-108 weighed roughly 100 + pounds and was primarily made up of stamped parts. While some machine gun systems were heavily machine from muzzle to butt stock, the MK-108 had 80% stamped metal parts.
The MK-108 was mounted in several different Luftwaffe aircraft, most in forward facing configurations with some MK-108s fixed vertically to fire on the belly of Allied bombers as fighters passed underneath. Post war British testing of the MK-108 showed devastating effectiveness, capable of easily splitting wings and fuselage with obvious lethal effects. The Luftwaffe estimated four to five MK-108 rounds could bring down a B-17, a quarter of what other German aircraft weapons would have to land to achieve destruction.
The MK-108 main short coming was its low muzzle velocity, between 1,300-1,700 feet per second, reducing its range and accuracy because of sizable round drop. For the weapon to work best, a pilot needed to get a quick burst on a target at relatively close range.
The comparison to the modern crew-served grenade launchers to the MK-108 are numerous, from general caliber effectiveness, weapon size and weights. It's not improbable to imagine an ambitious weapons engineer pull the MK-108 from German aircraft and reconfigure it for land use.