The decision to go for a rebated rim MAY have had connections with the Tray system of feeding to obviate a “rim jam” when feeding or retracting the case; But I think it was with a view to Export Sales of the Gun in truly Rimless calibres ( ie, 7,9mm, or 7,65mm or 7mm), simplified by simply making a Barrel change in the factory, and no other modifications to the Gun itself. The Higher Power of the 8x59 cartridge was a Tactical Consideration, and of course, Italy’s wanting to be “Different” and “Better” in comparison with France, Britain, Germany, etc.
The really good solution would have been to make the Breda M37 gun Belt Fed, using the excellent non-disintegrating M35 Belt used in the Fiat-Revelli M35 also in 8x59RB… Considering this “retreaded WWI gun”, it may be the rebated rim was a mechanical necessity for this gun, rather than the later M37 (although development of the Breda guns started earlier in the 1930s, and the M35 initial Breda Prototype was in “8x55 M35 Rimless Breda”.
The twenty round Tray was replaced by a 20 round vertical Box in the re-configured M38 Tank MG, also used widely (Bottom ejection) for tight space considerations in the equally small Italian Tanks of the late 30s and early 40s.
We may never know, uinless someone in Italy can research any existing Breda or R. Esercito (Artiglieria) Records, on the design and adoption of the Breda M37 and of the Fiat-Revelli M35 Conversion.
Saluti, da un Piemontese nato in Australia.