Experimental effects are usually applied per hit (e.g. a Burst Laser shooting salvos of three shots has a higher chance to apply Scramble Spectrum or to incur a power plant malfunction if the PP's integrity is below 80% than a Pulse Laser).
In the case of Feedback Cascade however, I believe this does not make much of a difference, as the effect is not a chance but damaging the charging Shield Cell Bank and shield energy restoration. The damage ist then just spread through the individual shots. This is in proportion to the actual Rail Gun damage, a 2B Rail Gun requires fewer hits to completely stop an SCB than a smaller 1D.