View Full Version : AR-15 bolt on M16 lower

February 7, 2013, 08:00 AM
I'm working a deal on an M16 lower, which comes with a 20-inch A1 upper. As much as I love retro guns, I plan on using the uppers I already have for my SBR.

Will I be able to slap these uppers with AR-15 bolts in them on the M16 and have them run full-auto with the M16 lower, or will I need to get an M16 BCG?

I've found plenty of info on M16 bolts in AR-15s, but nothing on AR-15 bolts in M16s.

February 7, 2013, 06:37 PM
Your uppers need to have a "sear relief cut" to work with the M16 auto sear.
Do a google search on "m16 sear relief photo" to see the difference between an AR15 uppper and a M16 upper. You'll also need a large pin to small pin adapter if you AR15 uppers use the large pivot pin.

February 7, 2013, 07:10 PM
Thanks for the info!

Your uppers need to have a "sear relief cut" to work with the M16 auto sear.

Will that affect the reliability of the bolts with my AR-15 lower? I've never had a jam with them, so I'm a bit nervous to tinker with something that works. But then of course, ripping through a 60-round mag of 5.56 with a 7.5 inch upper makes it worth the risk. :D

Would that also run me afoul of the constant back-and-forth argument about having "M16 parts" (a modified bolt) in an AR-15 when I'm shooting my SBR? Or would that only possibly (not trying to start a new front in the argument) apply to having actual M16 parts in it?

James K
February 26, 2013, 12:47 PM
"working a deal on an M16 lower"

I took that to mean a perfectly legal transfer of a registered machinegun. If that is not the case and the lower is an M16 lower, it is a machinegun and if unregistered is illegal no matter what parts are in it or what upper it has on it or what parts are in the upper.


February 26, 2013, 04:16 PM
AR-15's come and came with all kinds of bolt configurations. In order for an M16 lower to run full-auto you must have a full-auto bolt carrier. Some AR-15's came with a full-auto bolt carrier, many came with semi-auto bolt carriers and those come in several configurations.

Like someone said above, you will also need to make sure the Upper receiver has the auto-sear relief cut in it. Most aftermarket uppers are made this way. Many (maybe all) Colt AR-15 upper receivers do not have the auto-sear relief cut.

If the M16 lower is an older Colt type (and I'm not sure if anyone other than Colt was making M16 lowers prior to '86) then you will need an offset pin to marry up the older M16 lower to a newer flat-type upper receiver. This is not a big deal - you should be able to find a pin set for this purpose.

Other than the offset receiver pin, full auto bolt carrier and relief cut in the upper receiver, there really is nothing else to worry about. You're ready to rock-n-roll.