Model 2 or 3 versions of the SAW, one with a box mag, one with an M16 mag, and one with no mag (optional), then in the config for your SAW weapon, change the model= part to your box (or no mag model if you did one). Then in the M16 mag cfg, use the line modelSpecial= and add the model of the version with the M16 mag in it.
*Also, be sure to do the same with the SAW mag if you used the optional no mag model.