The old British standard is the 96/1 rule. Or 96 oz of gun to 1 oz of lead. That equates to 7 1/2# for 1 1/4oz shot or 6# for 1 oz. Of course, today's soft recoil pads can up that a bit or an automatic should you be a heathen.
Of course, serious shooters can go higher. A friend and his wife are BOTH in the Trapshooter's Hall of Fame. He uses a Krieghoff which I imagine is 8-8 1/2# and uses 1 oz and 7/8oz for doubles. I imagine the 7/8 oz is for the first shot. Of course for the hundreds of rounds shot in a day, that is a good idea. He has over 600,000 rounds out of that one gun. He sends it in every two years to be rejuvinated.
I think the Rule of 96 makes good sense for volume shooting. After all, it was devised with driven shooting in mind. Back then, your standard Brit game gun was a 2 1/2" 12ga rated for a 1 1/8 oz shot charge. So a gun that weighs around 6/12, give or take a bit. (Eventually, the standard Brit game cartridge ended up being more like 1 1/16 oz.) If you're shooting several boxes of those over the course of a driven day, it's a good idea to have a gun that's heavy enough.
However, that rule is pretty much out the window these days. Back then, all loads of a given shot charge were very close to identical in terms of velocity. But that hasn't been the case for a long time. There's a big difference, in terms of recoil, between a light 1 1/8 target load at 1145 fps and one that's moving 1400 fps.
And hunting as we do in this country, where you may only fire a few shots in a day to get your limit of, say, 3 pheasants, you can certainly stretch the rule of 96 a lot farther than if you're shooting several boxes, or 100 sporting clays targets. I don't have any problem shooting RST's 1 1/4 oz pheasant loads at 1200 fps in a gun that weighs around 6 1/2 pounds (although I usually shoot lighter loads than that), even though that old Brit rule says my gun ought to weigh 7 1/2. And believe me, if I were going to shoot very many 1 1/4 oz loads at 1500 fps through a double, I'd want one that weighs a lot more than 7 1/2!
