Quarter Circle 10 has made a name for itself as a high quality american made and designed manufacturer of pistol-caliber AR platform pistols and carbines. Their latest offering is a lower receiver that accepts MP5 magazines, and a side charging upper receiver.
Available in Spring/Summer 2017 here: https://www.quartercircle10.com
My own personal PCC is a complete QC10 build, and i can attest to its quality and reliability. Here i am in 10 degrees shooting Steel Challenge with my PCC SBR in Quantico VA.