I think you'll find that the 303 VIIz is dimensionally identical to the conventional ball rounds, fitting Lee Enfield, BREN etc. it WAS a higher power loading and marked ( with a black tip, and painted primer canneleur ? from memory ) not for use in the No 4 etc, an ‘Air service’ round and commonly used in the water cooled Vickers, along with the later VIIIz, an indirect fire round.
I wouldn’t dismiss these rifle calibre rounds too casually, remember eight of them, at 1200 rpm each, especially with the guns synchronized to 250 – 300 yds, the concentration of fire at closer ranges would do an awful lot of damage.
PA-28