Whats a good longer range hunting caliber(that this rifle comes in)? I already have a SAIGA 7.62x39. I need to shoot further and more accurate, maybe a little harder.
That rifle is listed as being available in the following:
.223 Rem.
.22-250 Rem.
.243 Win.
.25-06 Rem.
.270 Win.
7mm-08 Rem.
.308 Win.
.30-06 Sprg.
.257 Wby. Mag.
.270 WSM
7mm Rem. Mag.
.300 Win. Mag.
.300 WSM
.300 Wby. Mag.
.338 Win. Mag.
So, compared to your 7.62x39, that's a range of slightly less powerful to a WHOLE lot more powerful. Seriously...even a .30-'06 is not "a little" more powerful than a 7.62x39. You might want to borrow or rent one to play with at the range and see how much you like the extra power.
While every one of those rounds will kill a deer, most folks are more comfortable shooting at least a .243 for game that large.
What kind of hunting do you plan on doing? If you're hunting deer at any "practical" range -- where you have a realistic expectation that a decent hunter with some practice can reasonably expect to place a shot correctly ... let's call this 400 yds MAX -- any of the rounds between .243 and .30-'06 will do that job perfectly. "Most" hunters find their shots coming at well less than half of that distance.
If you're have some seriously wide-open terrain to hunt, and you will actually practice the art of longer distance shooting, some of the flatter, faster, heavier recoiling, more expensive rounds might suit you better. Those 7mm - .300 Weatherby, Remington, and Winchester Short Magnums won't be "necessary" to make shots at those ranges, but a slightly higher velocity makes for a slightly flatter trajectory, which makes range estimation a
little bit less critical.
Unless you're hunting very large, very tough animals at significant distances (which seems unlikely) I'd leave anything over .30 caliber out of the question.
Second, if I were to get one of those Weatherby magnum chamberings.. can I fire a standard load through it as well? Will it perform well?
Not sure exactly what question you're asking here. If you handload reduced charges or you buy "Managed Recoil" or other lightly loaded rounds, yes you can shoot the big cartridges at slower velocities and with less recoil.
You CAN'T chamber a '30-'06 cartridge in a .300 Weatherby Mag gun, if that's what you're asking.