The G17 has a 17 round capacity, the silence may be a weapon swap or reload.
It was noted by others he had purchased one recently as well as the rifle.
It’s noted by a couple of news outlets that he reloaded...at least once or twice. So even if you went the direction of limiting everyone to a five-round magazine...it doesn’t matter. The only real sure-fire way to fix this is muzzle-load guns or muskets.