After more than two decades of active service with the Brazilian Armed and LE Forces, the .45 ACP M.B.50 and M.953 sub guns manufactured by Indústria Nacional de Armas S/A during about the same time period, began to be gradually phased out from service as a result of the adoption of the 9x19mm caliber round as a local standard for pistols and submachine guns. It was just a matter of short time until several thousand INAs were in storage everywhere in military and police facilities began to call the attention of some people. Maybe they could be, somehow, upgraded/refurbished so that additional use of them could be obtained at reduced, affordable cost.
“Upgrading”, here, would mainly involve a caliber shift back to what had been, in fact, the original Danish Madsen M1946/M1950 design. It should be recalled that the Brazilian-made weapon collected a good number of detractors along its service life, to the point that the initials of manufacturer’s name (INA) were often referred to as “Isto Não Atira” (This Won’t Fire)! It gradually became apparent that most firing incidents recorded – feeding failures, mainly – were caused by the locally-made .45 ACP M2 rounds used and that were originally intended for the similarly-chambered M1911/M1911A1 pistols in use by the Brazilian Army. Tests conducted by several parties, including Companhia Brasileira de Cartuchos, the sole manufacturer, showed that the powder loading of that ammo proved unsuitable to regularly cycle the SMG due to its heavier bolt as compared to the pistols’. Complaints were also frequent about the weapon’s staggered-row, single-position feed 30-round box magazine that required higher bolt energy to feed the rounds into the chamber. CBC tried to overcome the problem with the introduction of the .45 ACP M4 round, nicknamed “Bezouro Verde” (Green Beetle) from the color of its lacquered bullet and primer seal, with a slightly more powerful powder load, but I recall that complaints remained: bullets would occasionally not exit barrels, and shelf life was extremely short. Anyway, the use of 9x19mm rounds in a “new” INA might prove appropriate.
The earliest transformation took place at Brazilian Army’s AGGC – Arsenal de Guerra General Câmara, Rio Grande do Sul State, in 1980. The sole prototype made, based on an INA M.B.50, used a 9x19mm barrel taken from a locally-made Beretta M12 SMG (aka MtrM M1972 in Brazilian service), to which a three-slot compensator was added to help controllability in automatic fire, keeping in mind that the original weapon had no semi-auto option. The staggered-row, two-position feed 32-round magazine, also from an M12, was given a slight forward rake to improve chamber-feeding characteristics, while the ejection window was moved from the right side to the top to match a new ejector position. The magazine release lever was enlarged but maintained the same (to the rear) actuating direction.
It is reported that the modified M.B.50 worked satisfactorily and met its general objective of being a concept tester. However, the officer in charge of the project, head of Arsenal de Guerra General Câmara’s R&D Department, was soon transferred to IMBEL’s Fábrica de Itajubá. He took the prototype to the new facilities, where additional INA 9x19mm transformations would take place. This will soon be covered here in TFB.
Higher-res pics here: https://imgur.com/a/Ew8JC