I actually found an M1 Carbine for a good price, which fulfills my "pistol caliber rifle" need.
It's a lot of fun! And cheaper than building an AR, too. When we were looking into building the 9mm or .45 AR, it was going to be expensive, and my husband was having a hard time finding an FFL in the area that would do the transfer for us. I didn't really understand what the issue was, though.
Unfortunately, it's really expensive to shoot the M1 Carbine. More expensive than the .308! And we're still working out the bugs with the cheap Plainfield (suspected slamfiring and FTE).