Patrick,
The loudest gun you can buy is a .50BMG, the big fifty cal Browning. Plain, and simple.
They are available in single shot bolt action. You can find full blown Browning M2 machineguns modified for semi-auto. Barret also makes a nice semi-auto.
However, it is $$$$$. A .50bmg bolt action can be found for the $1500-200 range. A Barrett Light is $5k+ A converted Brwoning is $8k+
Short of that, a .338, or 375H&H is pretty loud...
RobertN in Shingle Springs Calif