2. '6' for the first shot, '5+' for all subsequent shots at the same Genestealer.
If the SM spends his first action as a shoot, and rolls a 6 killing the Genestealer, thus giving line of sight to a second Genestealer. Can the SM shoot the second Genestealer at a 5+?
Yes for the Storm Bolter.
If on the SM turn he can shoot at whatever he likes within LOS and AP availability.
In overwatch he can only shoot the second revealed GS if it takes a subsequent action within 12 & LOS.
Sustained fire starts all over again for the newly revealed GS.