Doing a search here didn't reveal any recent posts on this topic, so I thought I'd check for updated info. For example: It's my understanding that the possession of fire control parts that can be used to make an AR15 operate full auto is not illegal provided one does not also have an AR15 that the parts can be installed in. (Please note, I'm not arguing the wisdom of not testing this law.)
I recently found that my Colt SP-1 1980 vintage has some M16 style parts in the fire control system when discovered I could turn the selector to the full auto position. But it does not have all the parts required for full auto. The parts required are the selector, the trigger(?), the disconnect, the hammer, the bolt carrier, and an auto sear (and optionally a burst fire cam).
Of these I have found that my gun does have at least an M16 selector and hammer. The disconnect I can't determine by looking down into the receiver (I haven't yet disassembled it). However it is missing the auto sear and the M16 bolt carrier, so it cannot operate full auto. I suspect the most it can deliver is an occasional slam fire.
I have ordered AR15 parts to replace these to be sure that I'm legal. But there remains some puzzlement. To start with, why would someone would put these parts in it without being able to complete the job? The lower receiver does not have the pin holes required for the installation of an auto sear.
The background on the SP-1 is that initially Colt provided a milspec receiver with the exception of the auto sear pin holes. Later, to discourage rampant modification, Colt added an auto sear block and enlarged the pins for the trigger and hammer so milspec M16 parts could not be installed. Then toward the end of the production, Colt stopped milling out the area where an auto sear is installed and went back to the standard pin sizes. This was all done for CYA, not to comply with ATF regulations.
The issues I'm pondering is what specific parts are banned and what constitutes a rifle capable of having these parts installed. My SP-1 does not have the sear block, and though the area of the auto sear is milled out, it does not have the pin holes to install an auto sear. This leads me to wonder if possession of this gun makes the possession of the parts illegal. As I understand, the ATF initially considered mere possession illegal but changed to also require the possession of a compatible gun by court ruling.
I've found research of this ponderous since complete information isn't readily available. For example, some sources seem reluctant to even mention an auto sear, while being quite open about the other parts. Others who offer original Colt M16 kits for sale (such as Sportsman Guide) state that they do not come with the lower receiver (of course), the disconnect, and the original barrel (a new AR barrel is substituted). But from the picture provided it's obvious that the hammer, bolt carrier and selector are all original (no sign or mention of an auto sear). Thus I deduce that the true illegal parts may be the receiver, barrel, disconnect and auto sear.
Since M16 components are openly sold with selector and hammers, I was considering the possibility that Colt may have at one time provided these parts in their SP-1 to consume their supply of M16 parts rather than milling new parts. This then would explain the presence of these parts in my AR without it being capable of an auto sear.
Can it be?
I recently found that my Colt SP-1 1980 vintage has some M16 style parts in the fire control system when discovered I could turn the selector to the full auto position. But it does not have all the parts required for full auto. The parts required are the selector, the trigger(?), the disconnect, the hammer, the bolt carrier, and an auto sear (and optionally a burst fire cam).
Of these I have found that my gun does have at least an M16 selector and hammer. The disconnect I can't determine by looking down into the receiver (I haven't yet disassembled it). However it is missing the auto sear and the M16 bolt carrier, so it cannot operate full auto. I suspect the most it can deliver is an occasional slam fire.
I have ordered AR15 parts to replace these to be sure that I'm legal. But there remains some puzzlement. To start with, why would someone would put these parts in it without being able to complete the job? The lower receiver does not have the pin holes required for the installation of an auto sear.
The background on the SP-1 is that initially Colt provided a milspec receiver with the exception of the auto sear pin holes. Later, to discourage rampant modification, Colt added an auto sear block and enlarged the pins for the trigger and hammer so milspec M16 parts could not be installed. Then toward the end of the production, Colt stopped milling out the area where an auto sear is installed and went back to the standard pin sizes. This was all done for CYA, not to comply with ATF regulations.
The issues I'm pondering is what specific parts are banned and what constitutes a rifle capable of having these parts installed. My SP-1 does not have the sear block, and though the area of the auto sear is milled out, it does not have the pin holes to install an auto sear. This leads me to wonder if possession of this gun makes the possession of the parts illegal. As I understand, the ATF initially considered mere possession illegal but changed to also require the possession of a compatible gun by court ruling.
I've found research of this ponderous since complete information isn't readily available. For example, some sources seem reluctant to even mention an auto sear, while being quite open about the other parts. Others who offer original Colt M16 kits for sale (such as Sportsman Guide) state that they do not come with the lower receiver (of course), the disconnect, and the original barrel (a new AR barrel is substituted). But from the picture provided it's obvious that the hammer, bolt carrier and selector are all original (no sign or mention of an auto sear). Thus I deduce that the true illegal parts may be the receiver, barrel, disconnect and auto sear.
Since M16 components are openly sold with selector and hammers, I was considering the possibility that Colt may have at one time provided these parts in their SP-1 to consume their supply of M16 parts rather than milling new parts. This then would explain the presence of these parts in my AR without it being capable of an auto sear.
Can it be?
I had just gotten the SP-1. I switched it to the fire position and noticed that it could be turned about 30 degrees beyond fire. 
My immediate thought was someone had installed the selector 90 degrees off. 
I was trying to figure out how that was possible when it dawned on me... the damn thing was uncocked, so I shouldn't be able to switch it to safe. I cocked it and sure enough, I was then able to turn it all the way forward to the real safe position. 
I looked into the receiver and saw it had an auto selector cam. I could have sworn I heard black helicopters hovering overhead. 
