Small scale "slaughterhouses" (multiple per state):
One or two workers kill not more than 5 horses a day.
Owners can't be paid for their horse. The horse must not see other horses killed. The worker throws grain on the ground or bucket (which has been cleaned, no smell of blood), the horse puts his head down, and the worker shoots him in the brain with a penetrating projectile for instant death. The owner is allowed to watch.
The carcass is gutted and frozen. A refrigerated truck will come weekly, which can stack many more carcasses together than the traditional truck full of live horses.