On average, you're not going to kill anyone with a single blast from a shotgun; drop them to the ground, yes, but not kill.
At worst, mooks have 10 boxes on their track. They're likely to have at least BOD 3, and wear some kind of armor of which the worst (Armored Clothing) is AV6.
You mentioned -5 DV and wide spread, which would put them at medium range and would have given them a -5 to defense. Assuming Wide Spread and Burst fire didn't stack, the average AGI 3 REA 3 is down to 1 die, or 0 dice if they do.
Given that the Enfield AS-7 has a maximum of 5 hits possible (6 if you swap the laser sight for a smartlink) unless you were using Edge, this means the targets have to resist anywhere from (13-5)+[1,2,3,4,5]= [9,10,11,12,13]DV with BOD 3 + AV 6 (or more) + AP 4 dice. This gives them a reasonable chance to, well, not die, at least.