We are Legend

[StickMan]

toward the beginning of the century you still have flintlocks and muskets being made, as well as the early breech loading rifles/bolt action rifles.

The Gatling gun was invented in 1865.

I've never seen anyone talk about a pump action rifle... perhaps you mean shotgun?

semi-automatic and fully automatic weapons start appearing in the 1890s

Pump-Action Rifles existed for a very short time as they quickly were replaced by Rifles with other firing systems, I didn't even know about them till I started digging around. Gatling guns might come up at some point but I'm not really looking for full auto weapons as that tec is a little later in the century than I want to go. The Pump-Action are going to be the top of the line experimental guns the PC's might find eventually once the story unfolds more.

Caros: The virus is air born, and takes some time to fully take effect unlike many "Zombie" Viruses. But also like I am Legend the Virus has spread over sea. The water thing does not really work as you still need supplies to live that you can only get on land.
The PC's will be dealing with gathering supplies and competing with other survivors, unlike I am legend the Immunity rate is higher. Cults and paramilitary groups in the area are going to be just a few of there problems.
As well they will be dealing with tracking down were the virus came from as they will soon be introduced to a small group trying to find just that out as well as how to cure or better fight infected.
Notably the technology of the 19th century is being augmented in many areas by magic meaning that some technologies are advanced beyond the capacity of the 19th century.

Edit: Posted while I was typing.
kpenguin: Not yet but likely will happen eventually. There may be a number of different Intelligences levels on pack leaders. I may even have the PC come in contact with Infected that keep all mental capacity, I'm not sure how these Infected will interact with the others however.