Quoted:
So why would I want to build a 9mm AR? I do like Glocks and reload so having a SBR that can use the same mags and ammo kind of appeals but I'm having a hard time pulling the trigger on a new build that's chambered in 9mm?
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One of the biggest reasons for a PCC is that many indoor ranges do not allow rifle calibers, but are OK with rifles/carbines in pistol calibers. That can be a powerful incentive for those living in places where finding places to shoot outdoors is difficult or just too uncomfortable for most of the winter or summer. Also, caliber commonality, and mag commonality in some cases. Used to be that PCC was cheaper to shoot, thus getting in more trigger time, but that's not quite the argument it once was, either.
If you've got a backyard shooting range, not so much - then the only reason to do it is for fun and "cuz, 'murica!"