JR719 is right, but I will add to it a little. They work really fine for me in my SKS but they do take a little practice. Push down near the rear of the rounds, just in front of the clip, and if they start to jam up going in just release pressure for a second then push again. In fact I use the thumb of my left hand for this. This usually gets them right in, not as fast a changing a magazine, but not bad.
Before you use them I suggest you make sure a round will slip through the clip without hanging up anywhere. I had to take a small file and remove burrs from mine. Before I did this they were a real bear to use, but that bit of work made a world of difference. Jim.