Pistol Caliber Carbines are definitely becoming mainstream-especially 9mm. Usually pistol caliber carbine uppers require dedicated lowers. Now, CMMG offers an AR Conversion magazine allowing you to use a mil-spec AR15 lower with their radial delayed blowback pistol caliber uppers.
I was excited about this improvement because now I can use my registered (infringed upon) SBR lower and a CMMG short barrel pistol upper without having to register another lower as an SBR or having to purchase a dedicated CMMG lower and wait several months while wasting another $200 to shoot something less unwieldy as a 16″ barrel. The same can be said if you are a lucky owner of a pre-86 machine gun lower. Or maybe you just want to save a little cash and only purchase an upper, it will still allow you to shoot PCC Radial Delay Blowback Rifle uppers like the Resolute.
Of course an advantage of the CMMG Banshee lowers is that they take Glock magazines. So if you are Glock shooter, you might prefer the Glock lower. But options are nice. And since uppers are not regulated as firearms you can get everything shipped right to your door and convert your .223 rifle into a 9mm carbine. Before this the only option of using a mil-spec lower with 9mm upper was using a costly or fickle magwell conversion.
The AR Conversion is available in a complete PMAG or just the guts that you can drop into your own AR mag stash (You can get PMAGS for about $8 here). This allows you to load up to 30 rounds of 9mm into a 5.56/.223 2nd or 3rd Gen PMAG so that you can use your mil-spec 5.56 lower with a CMMG Radial Blowback Upper.
The CMMG AR Conversion magazine will only work with the CMMG Radial Blowback Uppers such as the Banshees, Guards, and Resolutes. However the CMMG AR Conversion magazine is an adaptation of the Mean Arms Endomag that will work with traditional blowback Colt/Glock variant uppers.
Price is $45 for complete magazines or $30 for the guts to put into your own PMAG. You can find those here: https://www.cmmginc.com/product-category/parts-and-accessories/conv-parts/9_arc_conversions/