In carbon-14 dating, a certain type of gun or whatever is hot at an organism to measure the amount of carbon right? While the half life of carbon is known, how can you know how much carbon the organism has absorbed in their lifetime. Is it the same for all organisms? If one human absorbed 400000 and you test it and its 400, how can you tell that the organism had once absorbed 400000?

In radiocarbon dating, we know that the rate at which ¹⁴C is generated by cosmic rays in the upper atmosphere remains approximately constant over long periods of time. Radiocarbon is chemically identical to stable carbon, and is incorporated into atmospheric carbon dioxide. This carbon dioxide is taken up by plants in photosynthesis, and the plants are eaten by animals, and the animals by other animals...

In other words, within not very broad limits every living thing takes up ¹⁴C at a constant rate as long as it is alive. Because this radiocarbon is also decaying at a constant rate, every living organism attains a steady-state concentration of ¹⁴C. When the organism dies, it stops taking up radiocarbon, but what's already present (in, say, bone or wood) is still decaying. Since the half-life is known (5730 years), to a fairly crude first approximation the age of the sample may be established by comparison to the same mass of bone (or wood, or charcoal, or seashell, depending on what it is you're trying to date) of current origin.

That will get you the age to about ±1000 years, which is good for charcoal from a Neanderthal campfire or bones from a 10,000-year-old garbage pit. To get better precision, calibration by comparison to a sample of known age is required, because the details of ¹⁴C uptake will depend on a number of factors: cosmic ray flux, drought resulting in less plant growth, an increase in the amount of stable atmospheric carbon caused by a large forest fire or volcanic eruption, ... More recent radiocarbon dates, with precisions of ± a century or less, are usually calibrated based on tree ring data or a sample from, say, a tomb with the year of death written on it.

For more information:

• This MadSci answer discusses the precision or lack of same in scientific measurement.
• The Radiocarbon Web has everything you wanted to know about radiocarbon dating, and most of what you didn't want to know.
• The journal Radiocarbon maintains an excellent page of links, as well as a song which is more likely to tickle funny bones down under, since it's to the tune of "Waltzing Matilda," the unofficial Australian national anthem.
• A thumbnail sketch of various other radioactive dating methods may be found here.