This seems like a problem without a good answer. I have no moral issue with you shooting the dogs, I just wonder if it's worth the potential risk (potential violence, potential litigation). My gut reaction is that I'd try and find an animal rights advocate, or shelter, and see if they'd bring the animals in from what sounds like a dangerous situation for them. I'm not sure what your LEOs are like...you sound like you're in a small town and they might be more helpful. I'd try and give them the problem - you should be able to pass by safely and I think someone needs to take responsibility to resolve the situation.

Without making threats I'd also mention to the owner that he has liability for the dogs and that if something happens, you will be coming to him for damages. There might also be laws restricting the number of animals on a property and how they're enclosed - there might be license and inspection agencies you can try and involve.

I think it's the kind of situation where no one will act appropriately until the damage is done. Like another poster said, you also might be underestimating the difficulty in using a pistol here. Are you going to shoot while riding? I don't think that would work so well...but if you stop, I think the dog(s) are going to be on you by the time you've acquired a shot. I think you're at a disadvantage here and I'm not sure a pistol will really help. Some dogs are smart - they even use tactics in groups. I'd be very careful.