This is a response to the gentleman looking for an extended magazine for his BAR 7mm mag. No body offers such for regular sale. Browning and Winchester are subdivisions of Browning International, all owned by FN of Belgium. You can go to the Browning site and see that Browning International offers a great many models for sale around the world which we never see here. But they have to consider the laws of the lands where they sell when they design firearms. So no extended magazines from them. Same reason plus lawsuits here is the reason the sale of the 9mm Highpower was recently moved from Browning to FN.
A GOOD gunsmith could probably extend the mag from the bottom of the existing floor plate. You need the floorplate and original upper part with feed lips. The floor plate serves as the magazine latch which is usually right under the bolt on other semi-autos. However, it still has to swing from a hinge and this will make reloading difficult. You end up with more rounds but the same slow reload. A quick detach and ram in the next loaded mag just is not possible with the BAR.
I passed on the Browning Pump for the same reason.