This is probably pointless, just my observation.
From the limited look I got at Desmond's rifle, it looked like a type of AK47 that is configured to conform to California's assault weapon laws---it looked like it had that short (single stack) 10 round clip. Usually when you see an AK in movies or TV it has the conventional 30 round banana clip. I just thought it was strange under the circumstances. I doubt the people in charge of the Island would feel the need to adhere to CA's assault weapon laws, and back in the 60s or 70s, I believe the laws were very, very lax regarding these kinds of guns.
Probably just a convenient prop, probably no one gave any thought to it.
Either that or I saw it incorrectly (didn't get that great a view of it)

With this show you never know what was convenient prop use or actual hidden information ;) Of course time factor may be pointless since the hatch seems mostly frozen in the 70's yet Desmond was defintely out and about in the mid-late 90's. Not to mention the modernised wash machines from last episode that were in there.

At this point I can only confirm that you may or may not be on to something. Hehe.

There are a couple veriations. AK 47, AKS and SKS the sks is semi automatic and normally has a five round magazine.