Here is another short video that demonstrates some of the challenges that you run into putting more than 5 rounds of 7.62x39mm ammunition in a military specification 20-round magazine.

These images show the two types of Colt factory 7.62x39mm  magazines.

The magazine on the left is a modified 20-round magazine that is blocked to hold five 7.62x39mm rounds and has a riveted base plate that cannot be removed.

The magazine on the right is a purpose built 7.62x39mm 5-round magazine.