Sources:
https://en.wikipedia.org/wiki/Schwerer_Gustav
https://en.wikipedia.org/wiki/Little_David
https://en.wikipedia.org/wiki/BL_18_inch_railway_howitzer
https://en.wikipedia.org/wiki/2B1_Oka
https://en.wikipedia.org/wiki/Karl-Ger%C3%A4t
https://en.wikipedia.org/wiki/Paris_Gun
https://en.wikipedia.org/wiki/M65_Atomic_Cannon
