They both fire the standard 7.62mmx39 M43 cartridge, the same round as the AK. Since it's semi-automatic like the civilian-legal AKs, the biggest functional difference is the magazine capacity. The SKS loads ten rounds from the top, the AK takes replacement magazines from the bottom at 20 or 30 rounds each. The SKS also has a stripper clip machined into the top of the bolt, allowing direct loading from 10-round stripper clips. The AK requires a separate charger guide to accomplish this operation.
A few caveats: after approximately 1990, the Chinese SKS rifles weren't permitted into the US with the bayonet, the Russian SKSs were. the Chinese SKS pictured here was one of the last batches that came in the US in the late 1980s. Also, I recommend avoiding the aftermarket high capacity magazines that are readily available. They frequently encounter reliability issues from quality control, and also incompatibility with some SKSs. The SKS rifle was hand-fitted in the final stages of assembly, so parts do not interchange without some fitting. Also, there was a time when BATF(E) was going after SKS owners fitted with high capacity magazines when (or as) they were discovered. I don't think they were going door-to-door, but there may have been a few apprehended at gunshow displays. You should be aware for various reasons that the SKS was something of a
lightning rod between BATF and gunowners. It seems to have an irrestable tinker factor, compelling some owners to make illegal modifications including (but not limited to) folding stock modifications, adding bayonets, automatic fire and more. Follow this
link for more information.
Irrespective of which SKS you settle on, I would not advise you to buy one that has been modified, particularly if the modification is permanent and the seller has inflated the price to reflect this.
The SKS is best left alone, leave the modifications to guns that were made to accept them. Some accessories border on outright fraud, the replacement dust covers that serve as a scope mount come to mind. As any experienced shooter will relate, the scope must be on a secure, tight attachment point to hold zero. SKS dust covers are also fitted but have enough clearance to negate the effective use of a scope. Original SKS scope mounts are attached at the side of the reciever, and require modifying the stock. They also have a towering arm that holds the scope over the receiver, and bear a resemblance to the PU mount used on the W.W.II vintage Mosin-Nagant sniper rifle.
Another caution is the trigger mechanism. It's more complex than it appears at first blush, and should not be modified except by a gunsmith that specializes in that particular trigger mechanism. A lot of SKS trigger mechanisms have been ruined by amateur gunsmiths this way. Also, there have been some reports of so-called "slam-fire" caused either by
soft-primed ammunition or lack of
firing pin return spring. It's an easy fix, but should you discover this, do not allow it to continue unabated.
As far as accuracy, they are regarded as slightly better than AKs if for no other reason than they have a slightly longer
sight radius.
The two SKS rifles pictured above are no longer imported, and would probably be collectors items today. the Russian model would command a slightly higher figure since it has a laminated stock. The Chinese SKS would be lower, probably in the $150-$250 range. Prices vary wildly since they are entirely on the secondary market, and some models would qualify as "pre-ban", although the only difference would be the bayonet. The Russian SKS would likely be over $300.
The Chinese offered a plethora of variants, SKSs that could accept AK magazines, ("SKS-D"), so-called paratrooper models, sporting stocks and the like. Eventually you'll run into one of these platypuses.
The current batch are mostly from the former Yugoslavia and are reported to be very good. Prices can be as little as $120 to over $200. I've read these SKS rifles lack the hard chrome bore, and will require judicious cleaning after using corrosive ammunition. Irrespective of which SKS you settle on, I would not buy one that has been modified, particularly if the modification is permanent and the seller is charging extra.
Relevant links:
Simonov.net (collecting, info)
surplusrifle.com
Wilkipedia
World guns (Russia)
SKS Man (parts, acc'y)