Excellent comments, Jonty.
Re: trigger gauges.
I now have an electronic pull gauge that cost $50-60. It works fine, and as Jonty says, there's error in every tool and technique, so you need to average a bunch of measurements. To me, the biggest plus to the gauge is it's portablity; it fits in my tool chest and I can carry it to the club easily. But it is certainly no more accurate nor precise than a dead-weight pull that Jonty describes.
If you have a good scale that measures in the range you need (for years I used a $10 postal scale)a dead-pull is the way to go. I use an empty coffee can with 2 holes punched near the top. I attach a string to one hole, thread it through the trigger guard and attach it back to the can, which sits on the floor. I add a couple pounds of lead ingots, then dribble in some cast bullets. Holding the gun by the barrel, straight up, perpendicular to the floor, I slowly and steadily pull up until the can clears the floor and/or the trigger breaks. Then you can weigh the can to the 1/10 ounce if you desire. A little experimenting will teach you the gun is often not perpendiculat to the floor. That is, the correct direction of pull is not necessarily parallel to the axis of the bore, but often angled up toward the heel of the butt.
This may seem a little primitive, but it's actually more precise than gauges. Not that you need that precision for shotguns, but I can measure varmint rifles to fractions of an ounce.
