So many anglers use the same shallow rigs for carp and F1s but they are very different creatures and require alternative set-ups. F1 bites are notoriously difficult to hit but placing all of your shot in a bulk just above the hooklength helps create a bolt rig effect. The fish hit the hookbait at pace and the hook is set thanks to the weight of the bulk pulling back. A bulk also makes a noise when slapped on the water which attracts F1s.
The bulk of shot creates a bolt rig effect to help hit bites