Yeah... lemme guess.. box modeling? You appeared to have only payed attention to the side reference instead of the other references you can easily find by searching google (perspective, top bottom shots).
Lemme give you a list of things that should be redone. (note, this br would work pretty well for a rts)
Rails: There's two rails at the front that merge at the ammo counter housing. It also needs to be smoother at the front top part (which now is a sharp bend), bevel it a few times.
Handguard is plain wrong, it doesn't start there. it starts up a bit higher.
Barrel housing, square, undetailed. It lacks the curve from the BR and the holes.
Scope, is undetailed and completely wrong.
Barrel, needs 3 segments, that are extruded DIRECTLY out of the barrel. The rod under the barrel should be bigger and is supposed to touch the barrel itself.
You lack the triangle indents on the side as well as the charging handle.
From what I can see from the wire frame, there is no separate magazine either.
Some separate objects could have been extruded from the main geometry instead of being modeled separately and stuck in (trigger grip)
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