In Challenge 1 for the assault rifles all you have to do is hit the middle of each target and drop all the targets. Each target will take 3 bullets and then disappear. Remember this to keep your multiplier up to be able to score gold on it.
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In Challenge 2 for the assault rifles there are 4 targets that move across the screen one by one. They move fairly slowly. Aim for the middle and you should be able to get gold. You can shoot the back of a target without losing the multiplier.
In Challenge 3 for the assault rifles the targets are moving and they only take 3 bullets each before they disappear. Aim for the middle and only shoot 3 bullets at each target. Keep your aim straight to avoid loosing the multiplier.