''He did not understand this so I had to unload both the magazines on the table through the breech.''
''Why? Why not unload them by hand?''
The SLR magazine held 20 rounds. To check it was full, you pushed down on the top round. If the contents only went down a fraction so that there wasn't enough room to load another, it was full. In the Army unit I was a member of, we never filled the magazines to 20, instead only loading 18 rounds. This is because when full of 20, the spring at the bottom of the magazine was compreesed fully and had little springness left. This meant, the first two rounds had little upward pressure making a jam much more likely.
And don't forget to count your shots!