The whole idea of an "Ultra Mag" is long range accuracy. Simi-autos, by nature, are not as accurate as bolt actions. If you want an "Ultra Mag" get a good bolt gun, if you want a "simi-auto" get a 30-06, or some other "mid range" calibur.
As far as what make to buy, get the one that you like best, Browning, Sako & Remington all make good guns. The Sako is probably the best of thoses three.
Don't put a muzzel brake on it until you have shot ot a few times, you might not need it.