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Well, let’s see:

There’s a bit more material milled away at the top of the upper receiver. That’s probably not that difficult, but you need to know what to mill away. Oh, and you’ll need a mill.

The bolt carrier group in the M16/M4 has more mass - ie, there’s more material removed from the BCG on the AR-15. The additional mass of the BCG in the full-auto version helps slow down the cyclic rate of the rifle.

Difficulty: Well, you’d have to either find a M16 BCG, or add mass back to the AR-15 BCG. Adding mass back to the BCG could theoretically be done by making the additional sections of the BCG, filing them to fit, and assumes a skilled TIG welder to integrate them onto the existing BCG. Oh, and then we’re going to have to re-heat treat the BCG. I’d rate the difficulty as “high” without assuming a purchase of a M16 BCG.

Then there’s the changes in the trigger group. Let’s see if I can remember them all:

1. The tail of the trigger will be opened up in the center of the channel to allow the disconnector to ride through. Difficulty: Not that difficult with a small round file.

2. The disconnector has a “tail” on the M16 that just isn’t there. You’d have to find a new parts, or, make the tail and re-shape the existing material on the AR disconnector. Again, we’re assuming a skilled gunsmith with files and a TIG welder.

3. If you want burst-fire, then you need to find the burst disconnector.

4. The selector lever - you’d basically need to find a new one. Attempts to modify it for full auto would probably destroy it.

5. The M16 hammer has a “reverse hook” on it at the rear, to catch the full-auto disconnector. You’d need to make a new hammer or buy one. Welding on M16/AR-15 hammers and triggers will usually result in a molten blob of snot instead of a piece of metal.

I *think* that’s it, but I could be wrong. I don’t play with NFA stuff too often.