Terrorists chasing a President by dbing him with Spas is way more powerful then a President using a rifle. It's definitely imbalance but instead of balancing it we can make it more better.
Obviously the combat shotgun will be more powerful at closer range, but the usable range drops off pretty quickly. If the President, his driver, and the terrorists are all on NRGs, then the game turns into making sure the terrorists don't get close enough for the shotgun to be effective. It's not difficult to shoot out tires at distances shotguns can't match with a weapon that can shoot as far as the rifle can, so the shotgun is unlikely to be that useful.
I'd be fine with the rifle being a pickup that the President has to make a conscious decision to go to, if he wants to incorporate it into his strategy.