Conservation of Momentum - Rationale for ignoring internal forces in a system of particles
Internal forces in a system of particles will exist in pair & will be equal and opposite. (By Newton's third law)
So, in calculation of acceleration of center of mass, each pair of internal forces will become zero or cancel each other.
Therefore, internal forces of a system of particles can't impact momentum or acceleration of center of mass. So, they are ignored.
For example: If we consider a system of two particles, any internal force between them such as gravitational, electric or magnetic will not impact momemtum or acceleration of center of mass of above system of two particles.