I doubt that in anything like a normal load, the "jet effect" of gas from the muzzle is "most" of the recoil. It varies with the load, but usually runs around 25-30%.
The fact is that the "jet effect" is not something magical. Recoil (movement of the rifle to the rear) is the direct result of the movement of the bullet and gas to the front. Recoil is usually said to start when the bullet begins to move, and that is good enough for all but the most picky, but some gas always escapes around the bullet before it begins to move (it is that gas that causes throat erosion), and that gas becomes part of the "ejecta" that causes the "jet effect".
But the gas is the result of burning powder and its mass is the same as the mass of the unburned powder. In a handgun, the mass of the powder is a fairly small fraction of the bullet mass; for example, a 230 grain bullet with 6 grains of Unique is a common .45 ACP load. The bullet weighs almost 40 times the powder, so the bullet contributes most to recoil. But in a .30-'06 rifle, a 150 grain bullet might be propelled by 50 grains of powder, so fully 1/4 of the moving mass is gas. No means of redirecting the gas (by a muzzle brake, for example) can reduce recoil more than that percentage.
True, the gas escaping around the bullet and after bullet exit moves faster than the bullet because it is lighter than the bullet, and that makes the recoil proponent due to the gas a bit greater than it would be otherwise. Still, the total mass of bullet and gas adds up (in the rifle example) to 200 grains.
That recoil is basically fixed, no matter how the rifle works. So why would a semi-auto rifle seem to recoil less? Because there is the time factor. The recoil is the same as with a bolt gun, but the recoil is spread out in time. The rifle does not begin to recoil immediately; it has its own inertia to overcome. A rifle like the M1 operates quickly enough that the recoil is not evident all at once; the operating mechanism spreads the recoil out over time, thus giving a "softer" feel to the recoil. The recoil is the same, given the same load and the same rifle mass; but the blow is less abrupt and thus feels lighter.
That is why we use terms like "perceived recoil" or "felt recoil" to describe what the shooter feels, versus what physics tells us the recoil is.
Jim