I have been asked through the years which weapon system to concentrate training upon when such a choice exists. Unequivocally it would be the handgun.
For over two and a half decades, I have been involved in defending in the courts individuals who needed to use deadly force. Only a very small fraction of these cases involved the shotgun or rifle. The extremely high 90 percentile involved employment of the handgun.
Within a uniformed deployment, the handgun is oftentimes the only weapon system on scene.