[Slamfire]

I do not understand how you could a double in a match where you fired 75 rounds single loaded into a magazine but only fired sighting shots from the magazine.

What type of match were you shooting in? I does not sound like the Highpower Across the course match I am familiar..

Were you shooting the rifle on a bench? Did the doubles occur when shooting from the bench, or off a bipod, from the magazine?

If you were shooting from a rest or a bipod, shooting rounds from a loaded magazine, then the most probable explanation is a double due to bump firing.