How is momentum reduced?
In Physics, force = mass x acceleration. If the force is the same, then changing the mass must cause an inverse change in the acceleration. Bobbing the hammer reduces the mass, so that means the hammer will accelerate more/faster when the same spring force is applied. At the other end -- when the hammer impacts the firing pin, the force transferred to the firing pin and then to the primer remains unchanged.
And momentum:
A quantity used to measure the motion of a body, equal to the product of the body's mass and its velocity.
Mass x Velocity.
If the velocity is not increased enough to compensate for the removed mass the momentum is reduced.