Two things. First, this is known as the "grandfather paradox" and there is no reason that the timeline won't shift back and forth between the two and still appear seamless to folks in those timelines. Second, the blunderbuss was about 200 years out of date in 1855.
I said no questions about the blunderbuss (LOL). I knew it was way out of date by 1855 anyways. As for the Grandfather's paradox what i was trying to say was that if the changes would be as seamless as you say then the continuous loop inherent in it would basically lead to all affected parties simply ceasing to exist. They would constantly be popping in and out of 'seamless existence' in a perpetual loop, and if people did not notice any changes they would simply cease to exist by the start of the second cycle. A crude analogy would be like making a photocopy of a photocopy, with each copy being a less clear iteration of its master ....with the only difference in this case being that the next full iteration basically fades completely.
Which is why (before this article at least) the consensus among most physicists was that if time travel would basically embody travelling into what could best be described as paralled realities. That is the only was the present reality could stay in situ after one cycle. And while the whole 'multiple dimension' thing had its own set of flaws it at least allowed fluidity of motion when it came to time travel.
Going back to the blunderbuss ....hey, not many people in the last couple of centuries can claim to having had their lights switched off with a blunderbuss.