[@Ollumhammersong]

The 40K wiki page also states "The weapon is based on an extremely ancient design, making it too heavy to be carry manually"

For comparison, the German MG 08 machine gun from world war one weighed 26.5kg without a tripod or water.

It's relatively low damage can be explained by the fact that stubbers are outdated weapons based on real-world, modern day technology while autoguns are modern by 30-40M standards, firing high-velocity caseless rounds.

But yes, GW is known for dropping the ball on logic in some cases, such as giving 40K tanks thinner armor than many late war World War Two tanks, or how no one is really sure what the deal is with Hellguns vs. Hot-shot Lasguns.