Does anyone make a round nosed .223 cal bullet??? I haven't kept up with AR stuff.
If I could find a cheap/reasonable Luger barrel extension ["receiver"}, I would try a .22 Luger. I'd make the cartridges out of .223 Remington brass in a shortened RCBS .223 die set, like I did with the '256 Mauser'. That allows me to use a .223 (or 5.56mm) chambering reamer, which I already have. And AR-15 5.56/.223 die sets are cheap on eBay.
Even if no one makes a round nose bullet, it is not hard to make up a set of aluminum blocks to hold the pointed bullets while you mill them to a blunt nose. I've done similar work with 30 cal carbine bullets, but on those, I drill out the base to lighten them for 30 Mauser.
An amusing project. Sellers on eBay seldom separate the Luger barrel from the extension, and they seem to believe that a Luger barrel + extension is worth ~$400...
Maybe I'm living in 1985 but I won't pay that much for a barreled extension...