I've done the 7.5" flamethrowers, the 11.5" commando uppers, the 14.5" M4's, and the 16" in both standard and dissipator uppers. I've even done the Colt 633 DOE and 635 9mm uppers, but I keep coming back to the plain Jane 20"M16A2 in 5.56
The shrike looks like fun, and the LM-7 looks like it could be a winner too.