Thats for the old HK G11 which is a caseless round. The G11 platform never got past the testing stage and the 4.92mm round didnt perform so it was dropped
This ^^^^ is what I saw on Google when I searched for it. The other rounds were similar to FNs 5.7, some closer to .30 carbine length than a small pistol cartridge.
The MP5 is on the way out as most professionals now prefer a PDW that fires something close to, or an actual intermediate rifle round like the 5.56mm.
If you want to carry a pocket rocket, look at the Heizer Defense 5.56x45 (or 7.62x39). If you don't get them with the bullet, the fireball will render them poop-less.

Personally I think that the R9 gets as much out of its design as possible.
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