The production of protons in heavy ion collisions through the knockout mechanism (abrasion) is described using the Glauber model. The multiple knockouts from the projectile, including the inelastic collision series with the target, are considered using a closure approximation in treating energy conservation. Calculations for reactions of ¹²C projectiles with several targets at energies of 1 and 2A GeV are compared to experiments. For large secondary proton momentum a strong dependence on the target mass is found and attributed to multiple scattering of the projectile knockouts.