I am wanting to build an AR SBR. I thought about a 10.5" MK18ish clone but I have an oportunity to get a POF 7.25" PDW upper. I don't see myself taking a defensive shot past 30 yards or so. Would the POF have enough velocity for fragmentation with FMJ's or would I have to run SP/HP ammo with it? Should I just go with the 10.5" and forget the POF? Anyone with a 7" upper please chime in on if is actually usefull or is it more of a toy.