Serving warrants in crack houses means very close quarters, short time frames, and plenty of backup.
When I asked that same question, I was told that the double was tres reliable,had some intimidation factor, was shorter overall, and one could Whippet out faster as needed. With all the backup around, having more shots was less crucial.Having a shot or two RAT now was more.
BTW, while memory can err, I recall no PG only equipped shotguns. The Whippets and others were capable of shoulder firing. IIRC, most of the doubles were Savages.
Whippets are cropped at both ends, minumum length bbls and a short but usable stock.
The guys also were strong on Para-Ord's with big mags and tritium sights, backup revolvers(The Charter Arms 44 was popular) and lots of Kevlar.
Last estimate I caught on the news, over 10% of B-more is addicted to something.It'll top 300 homicides by New Years....
HTH....