I'm just sitting here with various thoughts about your situation going thru my head. They may be dumb, but, their mine. ::
Depending on the leader of the pack, feral dogs can be worse than most other animals because they generally do not fear man.
There is always a leader of the pack. Taking him/her out will usually stop a attack.
Dogs can't read human minds, but they are very good at reading human actions and attitudes.
On more than one occasion, I have stopped large agressive dogs in their tracks by instantly taking on a KILL thought and attitude, concentrating only on the dog, to the exclusion of all other things around me. I really don't know, but I wouldn't be suprised to find the hair stood up on the back of my neck. That kind of attitude, nothing less will do.
The several times this happened, the dog stopped with his back hair still up. He looked at me while I glared at him. He slowly took a rearward sideways step and then moved away keeping his eyes on mine. I slowly backed up a half step and then proceeded on my way but not taking my eyes off of him until he turned away. These were large dogs...60-90 pound size. (This is not BS. I sware it is true.)
If all I had was a Philadelphia derringer for defence, I think I would see what else I could find that has more than one shot.
A load of 20 grains topped with a wonder wad, a .45 ACP case worth of #4 shot and a wonder wad over the top might do something to change a dogs mind. It couldn't stay loaded this way for long periods of time because the oils in the lower wad would contaminate the powder.
I doubt that that load would kill a dog unless he was less than 4 feet away when I shot it.
I sure wouldn't want to blind a dog by shooting him with a shot load that wouldn't kill him.
A .45 cal ball would do a number on him with a solid hit, but with the accuracy of a .45 derringer, I doubt that I could hit him.
If I wanted to use my derringer, it would be on a instantanious basis so it should be capped.
Half cock notches are far from safe.
I would hate to have the dam thing in my pocket and have it fire and blow 1/2 oz of #4 shot into my bleep because the half cock failed.
I would think the muzzle flash and noise of a .45 would scare the manure out of a dog if it was at close range. It would me if it was aimed my direction.
I think I might load a gun like that with 35 grains of powder topped with a wad of tightly rolled newspaper. That wouldn't wound a dog, but I would think it would cause the dog and its pack to rapidly change their mind if they were attacking me.
Another nice thing about the derringer noisemaker, If it misses, it won't hurt some bystander I was unaware of.
I think I would carry a nice 1 inch in diameter X 18-20 inch long nightstick sort of thing to back up my noisemaker.
I think with that set up and the ability to develop the attitude I mentioned above, I would feel safe.
Just dumb rambling. Sorry if it didn't answer your question.