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Most strings have a ball end, where the end of the string is wrapped around a hollow metal ball and then wound around itself. This creates a stop on the end of the string to hold it in place in the bridge.
Fender is the only maufacturer of the bullet end. The bullet is a small, tubular piece of metal that looks like a . . . uh . . . well, okay, it looks like a bullet! It is soldered directly to the end of the string. It was devised for the Stratocaster and allows the string to seat itself more firmly in the tremelo block.
Click on the picture above for a good close-up look of how the bullet end looks and how it seats itself snugly in the tremelo block.

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