The Classic Army MP5 lowcap magazine is a 50rds plastic lowcap that can be used in almost any type MP5 replica. 

Tips:

Use a speedloader to load your magazines. This allows you to load the optimal capacity of the magazine.
When the magazine is still new, the spring will have to be stretched a bit. So don't overfill the magazines the first few times you play. You will notice that as you have used the magazine more often you can use the maximum capacity without putting too much stress on the spring. This is conducive to longevity.
Make sure to completely empty the magazines after playing. This way there is no stress on the spring and your magazine will last longer.

Unfortunately, it can happen with every type/brand of magazine that it does not fit well or feed well in your replica. We recommend that you try a different brand of magazine. You can often find out on the internet whether a certain brand/type of magazine is compatible with your replica.