Hi, Vince and guys,
A little confusion here. A DIAS (drop in auto sear) takes the place of the auto sear in the full auto version, but requires no hole in the receiver. It does require an M16 type bolt carrier, selector, hammer, and the other full auto parts. It allows normal selective fire.
When they first came out, BATF did not designate DIAS's as machineguns. They later did, but those made before that 1981 ruling have no serial numbers and are not required to be registered or subject to a tax-paid transfer. BUT, if you have a pre-registration DIAS and an AR-15, you can be presumed to have the intent to manufacture a machinegun. (Why else have the DIAS?) This was deliberately left unsaid in the ads for those DIAS's that came out a while back. They ARE legal to own, but NOT if you also have the rifle. (It has been stated that the DIAS sale was an entrapment scam by BATF, but I do not know if that was ever proven.)
A registered DIAS is, of and by itself, a machinegun. It can be transferred legally only on a Form 4 with a $200 tax. Whether it is installed or not is irrelevant; it, not the host AR-15, is the machinegun. The AR-15 need not be registered. No more DIAS's could be made after the 1986 law.
A second device, called the Lightning Link, is also considered, of and by itself, a machinegun. AFAIK, all are illegal unless registered. Again, an owner of an unregistered LL is in violation whether or not he owns an AR-15 rifle. The LL allows full auto fire only, not selective fire.
There is a lot of info on
www.ar15.com, but one article confuses the DIAS and the LL.
Jim