Headspace is controlled by the depth of the rim seat versus the rim thickness of the shell. Either a break open or sliding breech gun can have normal or excessive head space. My 16ga Halifax which is basically an R type Darne which does not have the obturator discs except in the highest grade of the R series weighs in @ 2oz under 6Lb. This puts it those 2 oz shy of the 96 rule for an ounce of shot.
As I understand it the "Rule" was designed for full loads of black powder. It has been suggested when using smokeles, which has a lighter powder charge weight when loading to the same velocity the Rule can be altered to 90.
The fact that .410 is not a gauge has nothing at all to do with it. A normally loaded 2½" .410 could be fired comfortably from a 3Lb gun. It might not handle good but recoil would not be excesssive.
Altering the velocity of a load also of course alters the "Rule" as to appropriate shot weight. A 2 3/4 - 1¼ load would normally have less recoil than a 3¼ - 1 1/8 load from a 6 3/4lb gun based on the 96 rule for the 1 1/8oz shot because of its much lower velocity.
Burrard acrually had a much better rule than the 96, he recommended for sustained shooting, as in British driven shooting, to keep the recoil "Velocity" of the gun below 16FPS. While not an absolute A very close figure for this is obtained by multiplying the total ejecta weight (shot, wads & powder all converted to Lbs) by the muzzle velocity & then dividing by the guns weight. If this proves to be uncomfortable then just work to a lower figure than 16. This 16 rule will actually work out real close to the 96 rule for a given shot weight propelled at close to 1300 FPS.
As Smallbore said the "Only" thing the 96 rule applies to is Recoil & what is comfortable for one person may well be murderous to another.
