You can get a pistol caliber AR... Macon Armory did that for me... .45 D/I upper. That is an older magazine adapter, which uses H&K USC magazines. Macon did a new magazine/adapter, which still fits a standard AR lower. They have 10 and 20 rounders, which are steel. I did ask Rudy (owner) about 15 rounders, but not following up with him since I'm moving out of state... have the adapter already, will order 20s when I move. Going that route, or a 9mm with magazine adapter allows you to convert the rifle over to 5.56mm or anything else that doesn't use a proprietary magazine well. If you want any info on CX4s or SUB-2000 Gen 2s, I do have one of each. I love pistol caliber carbines, just for the fun factor. Nevertheless, they can hold their own for home defense.