I keep running across this idea and it started out as so absurd that I had to imagine a reason for it besides "MP5 cool, 40mm cool, MP5-40mm = Maximum cool". To make it make sense, I've creates an alternate set of events suitable for an alternate Cold War era.
So imagine with me, if you will, a Cold War where the US and NATO never adopted 5.56x45. Maybe the US adopted the FAL, Or the AR-10 passed trials, but 7.62x51 is the standard cartridge for rifleman and machinegun in the Western World. This has benefits for range and hitting power, but fully automatic fire is left to the general purpose machineguns and teeth-rattling emergency fire.
The European powers began the 7.62 adoption with rifle grenades, but these prove to be diffficult to carry and store. The 40mm stand-alone launcher allows grenadiers to carry more ammo, and rifle grenades are available for special purpose munitions that are not compatible with the 40mm format. Every troop is able to use rifle grenades and may carry a few, but 1 soldier per squad would be a Grenadier with a dedicated 40mm launcher, an ammo loadout and sidearm.
However, having the M79-type standalone weapon for grenadiers was proving a detriment. Inside of 15m the grenadier was limited to a pistol and rapid switching from long range and close range was a problem. Undermounting a 40mm on a FAL or G3 is possible, but just makes the heavy rifles even heavier.
In parallel, the militaries found our quite quickly that 7.62 NATO is a terrible answer for sub-machinegun tasks. So the various countries keep their SMGs for MP and rear area and non-rifleman tasks. This pushed development of delayed and locked action SMGs to lighten the gun towards the personal defense weapon, similar to a smaller M1 Carbine. The Heckler and Koch MP5 being the best of breed. These weapons proliferated to many duties that didn't need or couldn't support a full-sized rifle. Jobs like truck drivers and officers, radiomen and cooks and others would be issued 3-6 magazines and an SMG. Only those where space it as the most critical like pilots and armored vechicle crews rely on pistols.
Someone gets the bright idea that Grenadier is a non-rifleman position and mounts a 40mm under a reasonable SMG (see pic above). Since the MP5 is lighter then a blowback SMG it's the natural candidate for a lightweight close-in weapon to complement the more ranged fire of the 40mm. when the enemy is at range, the grenadier engages with the 40mm. When things are closer up, they can swap seamlessly to the SMG and engage with automatic bursts.
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