Ammo? Pick up the phone and place an order.

The FN plant in Belgium was tooled up to make LOTS of .303 ammo, much of it either for export or their own Lewis, etc guns.

The Italians also used the same round in a couple of their own MGs as well and made quite a lot of ammo for themselves and (pre-war) for export.

Not being used as front-line arms, captured .303 rifles (and MGs) would have initially been issued to second-line garrison troops or reasonably reliable auxiliaries or "allies".

They picked up few "Bren" Carriers during the Dunkirk Handicap. Enough to type-catalogue them and impress them into service as SP mounts for light anti-tank guns or, later, on the Russianicon Front, as remotely-controlled demolition vehicles .

In the African Desert, The Australians and Kiwis ran the "Bush Artillery", pretty much completely equipped with captured Italianicon guns and ammo.

The Germanicon Ordnance system must have been really "interesting" by 1943, Ditto the Japaneseicon, who had few qualms about redistributing captured equipment and using it until it broke or ran out of ammo, fuel, etc. The Japanese Navy and their "Marines" were big users of their exact equivalent of the .303.

During the previous Great Unpleasantness, the Turks took possession of a LOT of Britishicon and Frenchicon materiel. Their 7.92 x 57 conversions of SMLEs are a case in point: Use them for training with original captured ammo, then, as that dried up, totally rebuild them to use "standard" ammo..