now i know a few that ordered the 20" m16a1 kits lately.
due to the panic all of a sudden 20" barrels are scarce.
every once in a while a 16" m4 or LW profile pops up at midway or JSE for nearly retail prices.
one thing you could do is build a replica colt 605 carbine--
you would have to use carbine length gas tube and a low profile gas block -pretty easy to do for most here at gunco--