Here's tip for you Carbine lovers. I bought my second GI issue about 2 months ago. Don't know the origin, it was at a local Mass Gunshop 10 miles north of Boston.
It was a jamamatic. I was so disappointed - I was really down. But.....I figured that these were so simple that I could find the problem. So I took them both apart, one at a time sequentially. Nothing......looked obvious. Then I came to the Mags! The follower was in backwards in the new Carbine's Mag!
I didn't spot that the follower was in backwards when I loaded it. (Good thing to check when shooting a new mag!)
I searched the Internet to find out which way the spring goes in and I found an author that explained that the high part of the spring must go to the rear of the follower. That way when the bolt begins to push a cartridge.....the one under has more resistance to sinking down and causing a misfeed. There should be more up-push at the primer end of the cartridge. (I didn't know that.)
I don't know which problem was the culprit or if it was a combo but the Carbine no longer jams and I'm very happy with it. Hope the tip helps you some day.