SKS's are not known for jamming, even when abused or not cleaned. The biggest problem that i've seen with them over the years, is with mag swaps.
It sounds like your's needs to have the feed lips on the mags trimmed back a tiny amount. If the nose of the round is running into the frame under the chamber opening, the front of the round needs to be allowed to lift a little more. Use a grinding stone (Dremel, etc), keep the lips the same same shape as they are now and just make them shorter...front to rear. Grind off a tiny amount on both lips and then try it. Go slow. It's easier to remove some more metal than it is to put it back.
A second possibilty is that the chamber has varnish in it. Some types of 7.62 x 39 ammo is "painted" with varnish. If the chamber gets real hot, the varnish can melt onto the chamber walls. That'll make chambering and extraction difficult. That varnish will not come out with a normal cleaning.
If you stick with the original ten round fixed mag, it'll run forever. My first choice is one of the Red Star 20 round fixed mags. With a little tuning on the feed lips, those will run forever too. I have those on all of my SKS's and they're as reliable as the original ten rounder's...just with double the capacity. It looks and mounts the same as the original ten rounder....it's just a little deeper.
I tried one of the extended detachable mags and ended up throwing it in the garbage can at the range. Pure junk! If I want to use a detachable mag, I'll use one of my other rifles. Keep yer powder dry, Mac.
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