There is little point to buying sxs rifle in caliber .300 magnum. They cost "mega tonnage" and offer nothing in return when compared to high grade repeater like BAR (wonderful rifle that self-loading after first cartridge is fired until magazine runs empty). The only use for double rifle is stopping power of dangerous animals at 10 paces or less that is why dangerous game guides in some African countries really need them. Stopping requires heavy slug of 400+
grains at 2000+fps.
If cost is your deciding factor then why bother hunting? The supermarket has meat much cheaper per kilo than you'll get from hunting!
I have a Winkler (Ferlach) 300 H&H double rifle that serves me well.
