Perhaps one of the best pairing of primaries and sidearms is the FN P90 and FN Five-seveN. Designed by FN Herstal, they were made to fire the 5.7mm round. In airsoft both of these have their versions made by different manufacturers so there's no lack of options for players. Liberator Airsoft gets to shoot the Gas Blowback versions as they promote Olight weaponlights.