In sprint, players are in a very unstable status and it should be very hard to insert the mags precisely into magwells.
I sugget the players are only allowed to reload in stand still, slow walk and combat pace. If In sprinting then press R, players are forced to slow down to combat pace to finish this process, then can sprinting again.

For the reload of belt-feed MGs and rocket launchers, I suggest forcing the players to stand still to do so.