I remember as a kid seeing photos in various gun books, like Small Arms of the World, photos of the 15" M16 carbine, Colt Model 605, I always wanted one. Finally decided to build one.
The original 605 was nothing more than the M16 with the barrel cut back to 15", so that the flash hider was almost up against the front sight tower. This made for a nice short, handy carbine. Alas, the short distance between the gas port and the muzzle led to unreliable operation. The 605 predated the XM177E2 that everyone knows as the M16 or AR15 "Shorty".
The solution was to use a Bushmaster 16" Dissipator barrel. I installed a Dissipator heavy barrel (which is also available in a lighter M4 pattern) onto an old Colt "Slickside" upper. Pretty straightforward. The only modification needed was to section some of the inner heat shields in the forends away to clear the Dissipator gas block, as well as a bit of filing on the ends of a reinforcing rib on each side. Then screw on the later open three prong flash hider. The buttstock is an old A1 pattern with no trapdoor. I have not milled off the bayonet lug yet.
The overall weight is close to skinny barrel A1 rifle, even though the barrel is much thicker (HBAR dimensions). The balance points for the two are very close. The result is a very stable carbine with a solid feel, but it doesn't weigh much more than the A1. I'm hoping this will make a good iron sighted tactical rifle for putting lots of lead downrange in tactical matches.
EDIT: The actual weight is 10 oz more than the A1.
If you're not familiar with the Bushmaster Dissipator, the front sight tower has no gas tube. A read gas block is in the same position of a 16" carbine. This ensures reliable gas operation.
I have not shot it yet. Alas, I ordered the wrong offset pin to mate the large pin Colt upper to the small pin Bushmaster lower. Hopefully I'll have the proper pin shortly and can test fire it.
Colt Model 605