Sounds like you shooting over his head to me, here's a trick that I use for SMG's: Aim for the legs, as the weapon kicks up from the recoil it will drag your sights up to his torso and as mentioned previously fire in bursts of 3 shots at a time. Head shots with an SMG are wildly inaccurate due to the recoil. Not one person on this forum could land 3 consecutive shots to the head with an SMG on a regular basis.
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It's our wits that make us men