“If I remember my math correctly it’s about 14 kilotons per gram of antimatter. (That might be low by a factor of 2, but not further off).”
By E = mc2, when 1 kilogram of antimatter annihilates with 1 kilogram of matter the reaction produces 1.8×1017 J, which is equal to 42.96 Mt
A 150mm shell comes in at 50kg.
Assume you get about 1/10 to actually blow up before dispersal, and 1/2 of the mass would be the corresponding matter to match the antimatter.
(.1)(.5)(50)(42.96) = 106.725 megatons
Or roughly 7115 Nagaski bombs (may have slipped a decimal there — 711.5)
Hell, screw rail guns, let’s figure out how to make anti-matter in larger amounts and try one in Tehran.
But can slim pickens ride a rail gun round?