spitstickler, that's some nice shooting! Don't be surprised if the SMK 168 actually does worse than the cheap monarch. The saiga barrel is a 1:12 twist rate so it won't do well with heavier bullets. 168 may be ok though. Keep us posted on your results.
The rifling is for the 150g ammo that is so commonly used. Stay with it if you want to shoot heavier ammo buy a boltie that's rifled for it.
I'll happily admit that I know very little about Saigas and have never owned one, but I do know a bit about .308's and accuracy, and the above is not true. The vast majority of .308 bolt guns are 1/12, including my Remington PSS. The barrel on mine is cut to 20", and it's run thousands of 175's and some up to 190 without a hint of destabalization and an average accuracy of .6 MOA. I know guys with 16.5" 1/12's who have done the same with no problem. I won't argue that a Saiga might do better with lighter bullets, but it's not because the rate of twist can't handle it.
Damn, I just saw this response (need to keep up with these poll threads).
Bolt guns are inherently more accurate platforms. That locked bolt means most of the pressure is directed out towards the barrel resulting in a faster bullet exit speed than what you would get from a semiauto action. In a semiauto the chamber doesn't hold all the pressure and a lot of it is used to move the bolt to the rear.
So less spin would not necessarily affect your groups in a bolt action as much as with a semiauto weapon, where the round is traveling at a slower rate. This is also why some reloaders can get away with loading beyond max recommendations (at their own risk of course) and getting some really good results on a semiauto rifle but if they used those loads on a bolt action they would most likely get a chunk of metal to the face. The S308 prefers lighter round because it allows it to exit the weapon faster and thereby get to the target faster with the fewest effects from external conditions.