In the case of a shoulder fired rocket launcher the rocket's exhaust is not contained within the weapon. If you do not believe that the weight of the projectile is involved, shoot something like a 45=70 with a heavy bullet and compare it to a rifle of the same weight with a lighter bullet, the same weight of ejecta and velocity. Bottom line, your example and theory are incorrect. Certainly muzzle brakes work but comparing the recoil of the same rifle with extremes of bullet wight will also demonstrate my point.
In your scenario, there are inconsistencies that make the scenarios incomparable.
Shooting a light bullet and a heavy bullet in a different rifle to the same velocity is not a comparison, for a number of reasons, mainly because you may not get complete burn in both cases using the same quantity of the same powder.
This is not about the bullet, it's about the powder, and to properly compare, the bullet and rifle must be the same too.
You need the same weight bullet to the same velocity, with different powders, and to compare the felt recoil, it has to be the same rifle as well.
Changing rifles introduces unnecessary variables. Some rifles have terrible ergonomics, and enhance felt recoil. The 45-70 Marlin, for example, is the worst for felt recoil. It's too light, there is too much comb drop, and the stock is too thin. The .416 Alaskan from Ruger has incredible ergonomics and is very comfortable to shoot. You can shoot a box of factory ammo in one session and walk away smiling.
Shoot a .416 Ruger Alaskan with a 400 grain bullet at 2300 fps. The felt recoil will be less than from a Marlin 45-70 shooting 400 grains at 1600 fps.
Take the 45-70, load it with 400 grain bullets, one round medium burning powder, one round slow, load the same velocity, and you will feel a big difference, a difference that is far out of proportion to the increase in powder weight.
