Again, to shoot the BAR effectively in full auto, one must be in the prone position. A longer magazine would make that impossible. One might look at the post-war M14, FAL or G3 which were a bit of a compromise between a battle rifle and squad automatic weapon. Those again had 20 round magazines and for the same reason. Once you flip the fun switch they are uncontrollable unless the shooter is prone and you can't shoot prone with a longer mag.