The general consensus of opinion is that a shotgun trigger should be set at about 4lb.
Certainly anything under 3lb is dangerous, and anything more than 5lb is a handicap.
Now for this thing about double triggers, and the fact that the rear trigger on a shotgun (controlling the left barrel) is traditionally set slightly heavier than the front trigger.
I have always thought it was something to do with safety: maybe that the heavier poundage was a safeguard against an accidental discharge caused by the recoil of the first shot.
Or maybe it’s because of the “fumble factor” when selecting the rear trigger in a hurry.
Here’s a round-up of the opinions of accepted side-by-side experts: Charles Lancaster says 3.1/2lb front, 4lb rear; Sir Ralph Payne- Gallwey says 4lb front, 4.3/4lb rear; Sir Gerald Burrard goes for 3.1/2lb to 4lb front, rear 1/2 lb heavier; and Gough Thomas agrees on 3.1/2lb to 4 lb for the front, and stipulates 4.1/2lb for the rear. Only W.W. Greener, who was quite opinionated on most things, says “about 4 lb” without differentiating between front and rear.
But guess what?
None of the experts say why the rear trigger should be set heavier than the front.