HDR enabled maps (Dust2 mainly) will cause the crosshair to be almost invisable.
Disable HDR, and use the commands mentioned by Dirty-Bird.
And if you dont mind having a slightly oversized crosshair, you can use cl_crosshairscale #### to make the crosshair bigger or smaller.
cl_crosshair 100 will make them span pretty much your whole monitor, cl_crosshairscale 1600 will make them microscopic (depending on your resolution)
I would recomend about 600-800 for visability, though it won't look pretty and you may have to guess at where dead center is while spraying an m249.
You can also change the color of the crosshair. cl_crosshaircolor <R> <B> <G> <A> i believe is the line. cl_crosshaircolor "255 255 255 255" will make it white i think. I don't mess with the command too much so...play with it and see what happens.
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