A Western vampire tale? Wow, I thought I'd seen all the possible variations. Nice angle. Keep going. I'll keep reading.

Now, as for crits and typos:
- I wouldn't repeat "power" so often in the opening paragraphs. At least change this one: "...a few weeks prior to that first taste of power."
- Is a Winchester a shotgun? This sentence implies that it is. "She loved her shotgun and I loved mine."
- You use the word "gun" three times in two comsecutive sentences. You should try to get rid of one of them. "...I cracked it open and popped a pair into the chambers before going back out to give Grandma her gun." That could be stated "...I cracked it open and popped a pair into the chambers before going back out to Grandma."
-Beware cliches. You are too good with phrases like "The four horses snorted and stamped, shaking their heads and blowing hard to clear the dust from their noses" to let a weak statement like "on their merry way" sneak into your writing.
- No crit here. Just a compliment. I loved this simple statement: "felt a punch of adrenaline".
- Eliminate unnecessary adverbs. The phrase "some harlot's overly large bosom" is just as visual and more direct if you say "some harlot's bosom." After all, you already said "suffocating". We get the picture.
- Misspelled word here: "inerrupt".
- I think you meant "others" in this sentence: "...the other's began turning their horses..."

Overall, I'm intrigued. And that is the biggest hurdle in the first 50 words of a novel. Keep up the good work.