how about TAVOR, it's technically an AR but it's lighter than some SMGs, it would be like a K1A (which is NOT a SMG in real life, but a carbine like the M4). the stats are
name MTAR-21
ammo-5.56*45 NATO
weight-6.5 pounds (about same as a P90)
barrel length- 15in (slightly less than a M4A1)
comes with a reflex scope similar to that of the P90
it would be a hybrid SMG-AR, slightly less accurate than the M4 but more portable with a smaller moving crosshair