Genuine curiosity here: Have you ever fired an assault rifle?
I have. My take on them is: they suck. Just to wield and aim, they're awful. I'd much rather fire a good Winchester, Marlin, Henry or something of that nature. They're sleeker, more comfortable to wield, and in my experience far more accurate.
Just on the merits alone, I can't understand why anybody would buy one other than, as you suggest, as a substitution for a dick. But even then, if I were to not have a dick, I'd still rather carry one of these:
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Than one of these:
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