Shootstuff makes a very valid point. The best thing you can do for a "brush gun" is just build a very short, handy rifle. I wouldn't use any caliber smaller that 6.5 diameter as most of the smaller stuff is just too explosive. In your situation, just stick with heavily built 150 gr bullets in your 270 is about as good as you can do.

Rather than try to buy a rifle to improve your odds, maybe the one thing that would help is simply some field work in your hunting area to study the best locations to give you the optimum field of fire etc. In other words, change hunting tactics rather than rifle.