I've seen several "remade" M16's hit the market in the last few years. I don't know if they'll ever be a problem but knowing that they could be a problem is enough to keep me from even thinking about buying one like that. I've been collecting NFA since 1991 and started building AR's in 1988. I own an early AR built on a Sendra receiver. Every Sendra I've ever seen was made on an A1 forging. At the time Olympic Arms was offering to replace receivers, the forging that Sendra was made on did not exist. Also, at the time Olympic Arms was offering replacements, Sendra had been out of business for some time. The fact that this Sendra has one of the most recent forgings is a clear indicator that the receiver was replaced recently and is, yes, I'll say it, contraband. Someone thought they could get more money out of their transferable machine gun if it was freshened up and modernized. There are so many things wrong with this receiver that have already been pointed out, but to summarize, the forging didn't exist in 1986, Sendra did not replace it, the markings and stampings do not match the font of any other Sendra in existence, the IL is missing from the stamping, and lastly, Sendra always, yes always stamped SEMI and not FIRE for selector markings. This receiver is bait of the worst kind. I wouldn't want to own it under any circumstances.
There's another one out there that's just as bad. Someone took a Colt 9mm Sporter and "converted" it to a transferable H&R M16A1. And they did an absolutely horrible job of it too. These people really need to up their game if they're going to try to pass off obvious fakes. I'm not going to call myself an expert but I've been around the NFA world for a long time, long enough to reach RKI status. I just feel sorry for the sucker that ends up owning one of these. I also feel sorry for the item that may eventually be confiscated and destroyed. There are only so many left in the registry. We can't afford for this to continue to happen.