But is it?
Neutron Blaster Cannon II
Signature Resolution 400m
Antimatter Charge L
Assuming the charge is cyclindrical, volume = pi() * radius^2 * height. For the charge to have a radius of 400m it would need to have a height of 0.00000004973592 m, or 49.73 nm.
(even if you assume 400m = diameter and so a 200m radius, you still get 198 nm).