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He said the Ranger branded bullets used were designed to "expand and mushroom" and cause substantial damage.
Source: Sky News – Pistorius Throws Up During 'Graphic' Evidence

I think it means that it means bullet was in a way that would go fast in the body. Am I right?

6

[. . .] a bullet that will transfer its energy quickly onto its target. This is where soft-point and hollow-point projectiles come into their own. They are designed to "mushroom" or expand when they hit any resistance ([e.g.] animal flesh). This expansion makes the diameter of the bullet increase. This is done for two reasons: When travelling through flesh, this expansion slows the bullet down quicker than a bullet that doesn't mushroom ([e.g.] full-metal-jacketed bullet), and causes the energy to transfer in a shorter time span, causing greater shock. And the other reason is that when the bullet increases in diameter, it leaves a wider wound channel in the animal, causing greater damage and a quicker kill.

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Source: huntingclubshoalhaven.org, Which Bullets?

Basically, some ammunition is designed so that it deforms throughout impact, rather than retaining its point and passing cleanly through. This is actually designed to make the bullet go slow in the body, stopping sooner (imparting more energy at once) and harmfully widening the impact channel.

This is the sense intended:

mushroom
VERB
2 Form a shape resembling that of a mushroom:
the grenade mushroomed into red fire as it hit the hillside

      →2.1 (Of a bullet) expand and flatten on reaching its target:
         these are high-performance bullets which mushroom upon impact

Source: Oxford Dictionaries definition of mushroom

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Ditto to @TylerJamesYoung's answer, just to add:

"To mushroom" is an idiom -- well, probably more of a metaphor -- meaning to grow or expand very rapidly. We often say, "Sales mushroomed after the new advertising campaign" or "World population mushroomed when the Green Revolution increased crop yields".

Note this is quite different from the meaning Tyler refers to, "to assume a shape like a mushroom".

Some people refer to hollow-point and soft-point bullets as "expanding" when they hit the target, but this isn't really true. They don't get bigger: they flatten out. The diameter gets bigger while the length gets shorter. It's not inflating or magically increasing mass.

So in general if you said that something "expands and mushrooms", I'd understand you to mean that it grows larger. But in this case, it means that the diameter gets bigger and it takes on somewhat the shape of a mushroom.

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