The "standard" English formula for shotguns is gun weight about 96 times the shot charge weight.

For a one once load, standard for a British a 16ga, this works out to six pounds even. For an english 12ga game gun shooting a standard 1 1/16oz English game load this equals 6 3/8lbs. For "new" 1 1/8oz loads this is 6 3/4lbs. For 1 1/4oz pidgeon loads this works out to 7 1/2lbs.

If you survey a selection of guns you will find that weights vary some but not too much or too often from the rule of 96.

I haven't tried the formula for any rifles, or heard that it applies to rifles, but it might be a starting point to see if it works or if a selection of express rifles points to this or another formula.

JPK