I recently purchased a fully transferable ar 15 sp1 (registered receiver) and would like the forum's input as to the disadvantages, if any, of the ar15 versus the m16. Your asistance is appreciated.

No real disadvantages or differences, other than the fact that a factory M16 would command a steeper premium because it is a factory gun and not a piece converted by some other person or manufacturer.  The other obvious difference is the mag fence.  Most, if not all, SP1's were slabsides, but many of the conversion guns on AR15's (later model Colt's, Sendra's, Oly's, etc) as well as the later Colt factory M16's have the raised mag fence.  But for a shooter, there's no real difference.

Kolt did not make the SP1 mil spec. Didn't want lowly citizens to be able to make machine guns out of them, blah, blah... If it is converted, the only remaining diference than the mag fence is probably to front pivot pin. Does it have a screw? If so it is the large hole, and you will either need a kolt large hole upper, or a conversion pin.  Someone will correct me if I am wrong. I don't know that much about kolt. Don't like the politics of the company and won't buy them.  My M16 is a Rock Island, and it is mil-sec. HTH!  

Unreconstructed,
This is unrelated to the AR15 vs M16 topic but directed to your comment on Kolt. I agree 100%!! I just don't ever hear anyone saying it. Everyone started capping on S&W when the "agreement" crap went down with Clinton/Reno/Tony Blair in England against S&W and failed to recognize the fact that S&W still wants to produce firearms for the American Public. Where was Colt when this hit the fan?.....Hell, they turned tail and bailed from the civilian market and hid behind a rock. What a bunch of shmucks! I'm a handgun shooter/collector as well as a blackgun fanatic and as far as I'm concerned regarding handguns, Colt hasn't made a quality product in years, thier 1911's have been garbage compared to what Springfield, Kimber, et.al. have produced. Just had to comment!