[Grumpy]

If you want to use the pre-existing flash hider to pin and weld, have your MR556 barrel cut back to 14.7 inches. If you want to use a longer flash hider, such as the phantom, vortex, one of the suppressor mount flash hiders such as the Gemtech Bi-lock or AAC Blackout, Ops. Inc. 12, etc, it's ok to cut back to 14.5 and perm attach.

I did my own barrel work and found it to be much cheaper, although the names listed above do fine work and if you don't hae access to a shop, their services are well worth it.

I'd also suggest having the profile changed as well (either mid-weight or light weight) if you're going to go through all the effort to have it cut down. You won't be sorry.

 

[Grumpy]

Well, I don't know how sticky/picky CA gun laws are going to be if you have a non-full-auto lower mated to a full-auto capable upper. In fact, the bolt carrier for the 416 is different than the MR556, allowing for "select fire".

"Normal" bolt carriers are fine in CA.

The carrier does not "allow" for select fire. It is merely a part in the rifle.

There was some issue with this after Colt decided some years back to start removing the bottom section of the rear of the carrier. When they did this, it let some less then knowledgeable people to start saying that full shroud carriers were FA parts and thus illegal. This was wrong then, and remains so to this day.

The original SA carrier was designed this way as well and the fact that the 416 carrier retains this feature means nothing in the greater scheme of things. To be sure, HK's addition of the barrel extension "tab" and the cooresponding cutout in the carrier as well as its further cut back rear section are probably sacrifices to the gods of US "sporter" import regulations more then anything else. As it were when I removed my MR556 barrel for re-profiling, I removed the tab so if/when I get a 416 carrier, it will fit just fine.

Now if you were to start drilling extra holes in your lower and adding different parts to the lower, then this discussion takes a turn for the worse when discussing legalities.

But, since we're talking about non-FA lowers, any discussion as to the legality of the internal parts of an AR upper receiver is a moot point and a waste of time.