I realize that the BATF doesn't have a issue with using M16 carriers in the upper receiver so Colt uses them to simplify manufacturing and reduce cost which is a smart business move.
How is it that Colt can use M16 hammers complete with spur in their newer 9MM 6450s & 6951's lowers ?
Is it because Colt's lowers have the shelf area that isn't fully machined therefore creating an auto sear block ?
If that's the case then why not put in M16A1 triggers in them too since the only difference is the disconnector channel being open at the end ?
Anyway I was curious if anybody had answers.
I have seen that, and I would have to agree that Colt probably felt comfortable doing that because of the "sear block" in the receiver. If it weren't for that, all you would need would be a DIAS to make it a full auto. I had sent a letter to the tech branch about using M16 parts in an AR because I wanted to use the M16 trigger and selector in a Spike's Tactical lower so I could switch to the FA position, but still fire in semi auto. The response I got was something to the effect of: "We cannot give you the 'ok' to perform the modifications, nor can we tell you if substituting certain parts will make the rifle fire full auto, and that the law suggests you install AR15 parts or modify M16 parts to SP1 configuration. However, if you were to install the parts, and the gun fires one round per trigger pull, then it would be legal to make and own. If installing the parts were to make the rifle fire multiple rounds with a single trigger pull, it would be a machine gun and it would be an illegal conversion."
That seems to be the consensus. The ATF does not recommend any M16 parts being used (no surprise there) but legally as long as the
combination of parts don't allow your AR to go full auto then it's legal to use. Like for instance installing a M16 hammer or a M16-A1 trigger
they are full auto parts but they won't make your AR15 full auto by themselves.
Colt must feel safe with the sear block and that's why they continue to use it.
So did you end up using the trigger and selector? I've heard of people grinding down the auto selector cam (to be extra safe) to be able to flip it in the third position for novelty.
Well, what I ended up doing was grinding off the cam on the selector switch and using a regular AR trigger. The cam will interfere with the metal at the back of the AR trigger and not allow it to fire in the FA position if you don't. Since I also have a 9mm upper, I use an M16 hammer with the auto tab ground off on them. So, I went the "M16 parts converted to SP1 configuration" way to make it happen. All of my AR's that I build have M16 carriers for when I get an M16 lower/RDIAS.