The military disagreed.It had a grenade launcher and all the necessary accessories already in the gun, so it's worth it.
And you used kg twice BTW.
By the early 2000s, the weapon had settled on a design and was classified as the XM29. The XM29 was based on the HK CAW (Close Assault Weapon) (Cal. 18.5×76mm or 12 Gauge non-conventional). However, the weapon had serious problems: it did not meet weight or cost targets, and the 20 mm High Explosive Air Bursting (HEAB) did not seem to be lethal enough in testing. To compound matters, the kinetic-energy component had to be light and short in length. As a result, the 5.56×45mm NATO barrel had a length of only 250 mm (9.8 inches), which is too short to generate enough muzzle velocity to be effective as a standard infantry rifle. It was also too heavy and too large to be operated effectively by a soldier.