One of the best "Brush Guns" ever developed is the Browning BMG (Ma Duece). A 30-06 clone. Little heavy to carry and expensive to shoot, but will get the job done.
The H&K 40mm Grenade Machine Gun is also very effective...
350 rounds per minute and has a 'box' 32 round capacity... A little heavy to lug around (around 60 pounds) so those that prefer lightweight rifles might go with something else.
Here is a little video of one in action;
http://www.youtube.com/watch?v=hN4GlNZB1-g