What does flakvierlings mean in English?

Answer 1

Flakvierling is an anti aircraft gun.

Answer 2

This is an idiomatic translation and not a direct, word for word translation

From the German abbreviation of FLAK=FLugAbwehrKanone or "air defense cannon" and "vierling" meaning quad or quadruple.

Flakvierling idiomatically translates to Quadruple flak gun

The actual Flakvierling 38 was an antiaircraft gun mount used by the German military in WWII, it mounted 4 (four) separate, magazine fed, select fire, Flak 38 guns chambered in 20x138mmB, Long Solothurn ammunition.

The Flakvierling 38 had a theoretical rate of fire of 1,800 rpm (Cyclic) but since it was fed from 20 round magazines the practical rate was around 800 rounds per minute

800 rpm (Practical)