Angstadt Arms has a new AR-9 lower designed to work with a wide range of magazines. It features interchangeable magazine wells to use magazines from a number of pistol makers as well as different calibers.
So if you have, say, GLOCK 33-round magazines along with 30-rounders for an MP5 that you’d like to be able to use in a pistol caliber carbine format, the new AXL-9 lower lets you swap out mag wells to accept them.