The unthinkable: Converting M16A1's to semi-auto. Approx cost, involvement?


Matt G
March 27, 2004, 10:10 AM
My small-town P.D. just obtained three M16A1's from a DOD grant. My department wants to embrace the concept of a patrol rifle, but doesn't want the "liability" of full auto rifles.

Yes, I know. I completely understand and agree with the concept that we could train our officers on full auto, and the "problem" would go away. But The Powers That Be want SA, and a butchered FA rifle turned SA is better than no rifle at all, which is what we've got right now.

So my bosses want to know the approximate cost of converting the FA's to SA's. What all would this entail, to do it right? Understand, we can still keep the auto sears in our safe-- that's not the big problem. They just want to convert the rifles on the street to semi.

Your professional help is greatly appreciated.

If you enjoyed reading about "The unthinkable: Converting M16A1's to semi-auto. Approx cost, involvement?" here in archive, you'll LOVE our community. Come join today for the full version!
March 27, 2004, 10:30 AM
Howdy Matt,

The simplest way is to get some two-position selectors and install'em in the
rifles. The switch won't move to the full-auto position and cam the disconnect back and cam the auto sear forward. The only problem is that
a three-position selector can go back in in 3 minutes or less, and you've got
an automatic rifle again. Using the 2-position selector would probably entail using an AR-15 disconnector too. Since I've never done a reverse-conversion, I don't know...but it probably would.

To make it harder to convert back, you can go with a stock AR-15 hammer, which doesn't have a hook on top to engage the automatic sear, but it's
still an easy re-conversion.

Removing the auto sear is easy, but the hole will still be there...still an
easy switch to return the rifle to select-fire status. Heli-arcing the
hole will ruin the finish. I suppose that a properly sized pin could be heat-shrunk in the hole to make it more difficult, but still wouldn't stop a determined tinkerer.

As long as the officers are on their honor not to play with it, and there's
little chance of the rifles falling into the wrong hands, the 2-position selector and maybe the AR-15 hammer would be okay. Due to the
proliferation of gun show M-16 parts and drop-in auto-sears, there are
literally thousands of guys out there who could look at the rifle and
know exactly what to do to restore it to select-fire...and probably have
the parts on hand.

Check your PMs


March 27, 2004, 11:37 AM
I would assume the conversion is to be politically correct and minimize any lawsuits from the use of a "machinegun". Technically, if you replace the M16 hammer, trigger, disconnector, selector and bolt carrier, the rifle is a semi-automatic AR-15 (except for the extra hole). Since you don't have to worry about ATF, you could keep the M16 bolt carrier and change all of the other parts for about $40 per rifle. Probably cheaper with a LEO discount.

BTW, LAPD did the conversion on several hundred M16s. Might give them a call and see how they did it.

Badger Arms
March 27, 2004, 02:20 PM
Method A: To convert the rifle PROPERLY takes the following:
1) Mill the trigger, hammer, secondary sear, and bolt carrier. These are simple operations and can be done with a dremmel. Bushmaster has comparrison photos if you ask them, they will provide them.
2) Replace the receiver with a receiver made as a semi-auto.
3) Properly dispose of the auto-sear, sear pin, and sear spring. Tossing them in a lake or pond does a good job of destroying them.

Method B: There is a plate you can install between the pistol grip and the receiver that has a little leg which projects up in the way of the selector. It's still a machinegun, but it takes a little time to do. Since you've already got machineguns, I'd REALLY suggest you don't play with the parts and simple install the cripple device. It works as intended and was designed to keep trainees with little experience handling the gun from going full-auto.

Method C: The cheapest way to do it, though, is simply to replace the secondry sear with an AR-15 Sear. The secondary sear is what grabs the hammer and then releases it when the bolt carrier trips the auto sear. This part has a leg which protrudes back and is depressed when the selector is moved to the "Auto" position. You can even just cut this leg off. That keeps the auto-sear from doing its job and there is no mechanical way the gun will go full-auto. It would also be a good idea to remove the auto-sear and discard it as I suggested in Method B. Of course, it will still have to be treated administratively as a machinegun. If the department so desires, it can keep the machinggun parts (secondary sear, auto sear, auto sear spring, and auto sear pin) in the event it decides to sell the weapons in the future to allow their return to original configuration.

March 27, 2004, 02:33 PM
Howdy badger,

I'm tryin' to get a visual on what you mean by "Secondary Sear".
If you mean the auto sear that grabs the upper hammer hook...that's an M-16 item, as is the upper hammer hook. When the selector moves to the third position, the disconnector is cammed back out of the way, and the autosear is cammed forward on the bottom to put it into position to engage
the upper hammer hook. The AR-15 doesn't have that upper hammer hook.

Just for the sake of information, if the M-16 trigger and disconnector is left in place, the hammer will follow the bolt. The M-16 disconnector has a tail on the back that requires an M-16 trigger. the AR-15 trigger is closed on the backside, and won't allow the M-16 disconnector to be used...or at
least the triggers USED to be closed on the back.

The rifle can be converted to semi-auto only with just the installation of the
2-position AR-15 selector and Ar-15 disconnector...and it may require the selector only. Like I said...I've never reverse-converted an M-16, but if
the 2-position selector doesn't bear on the disconnector tail and move it back away from the lower hammer hook, only the selector will be needed.

Simply removing the auto-sear WILL prevent the rifle from doin' the Rock'n'Roll, but if the selector is accidentally moved to the Full Auto position, the hammer will follow the bolt because the disconnector won't engage the lower hook.


Badger Arms
March 27, 2004, 02:46 PM
I meant the disconnector. Comes from breaking down my Father's CAR-15 before I was old enough to drive. I'm a self-taught gun nut. Yes, cut the tail off the disconnector, remove the auto sear... that would do it but then you'd have two semi-auto positions. Replacing the selector as well would make the gun look traditional. Auto-sear has to be removed as I stated though. Here's a pic from the Bushmaster Site:

March 27, 2004, 03:07 PM
Badger Arms said:
I meant the disconnector.

Ah! Okay. I was scratchin' my head for a minute.:confused:

It's been over 30 years, but IIRC from the Marine Corps armorers'

The M-16 parts are the sear nose...on the trigger. The lower hammer hook
that engages the disconnector(Primary hook). The disconnector...The auto sear, which catches the upper hammer hook...and the upper hammer hook.

Now, I THINK that Colt calls the AR-15 lower hammer hook the disconnector hook(notch?) since there ain't an upper ammer hook.
The years make it a little fuzzy, but it's comin' back to me.

Yes...removing the disconnector tail would provide two semi-auto
positions if the auto sear was removed, so that would work too.

Interesting little rifle, an even simpler than the 1911...



Chris Orndorff
March 27, 2004, 04:57 PM
Our rifle armorer just changed about 16 M-16s to semi only. As I recall, parts cost $40-45 (as Hkmp5sd previously indicated).

March 27, 2004, 10:24 PM
Chris said:

Our rifle armorer just changed about 16 M-16s to semi only. As I recall, parts cost $40-45

Ayep...Selector, disconnector, and hammer (AR-15) and can probably do it
successfully with just the selector and disconnector.

Also, on the differences between the internals, the AR-15 bolt carrier also has a milled section that corresponds with the angle on the top rear of the hammer that captures the hammer in the event of a followdown reducing
the chance of a slam fire if the disconnector or lower hammer hook fails.
Neat design, if ya ask me....

MATT! Check your PMs. I've got the three AR-15 parts that I'll send to
your department armorer to try, but I gotta have an address.



March 27, 2004, 11:04 PM

March 27, 2004, 11:19 PM
Nice pix MP5...but the section of the carrier is underneath the firing pin.
The angle is cut to capture the hammer in the event of a followdown
so the hammer can't get to the pin no way nohow.
Works sorta like a morse taper...Wedges it up pretty tight. The carrier stops the hammer, and the hammer stops the bolt from goin' to battery. Neat.



March 27, 2004, 11:45 PM

March 28, 2004, 06:49 AM
Great Googa Mooga man! Are ya blind?? Right there it is!:D


If you wanna see it work, induce a followdown. Remove the disconnector from the trigger group and shoot the rifle. The bolt and hammer will capture each other. The bolt won't return to battery.


March 28, 2004, 09:09 AM
If you wanna see it work
I have seen the parts in person. I have one of both rifles. :D

March 28, 2004, 09:20 AM
I have seen the parts in person. I have one of both rifles

Me too...I've built/rebuilt/repaired a few...workin' as a smith for a Class 3 dealer and at the armory at Lejeune. Qualified as a "Field Expedient Armorer"...unofficially...and put it to use in the Land of Rice Paddy Daddies,
aka South Vietnam.

First time I saw the AR-15's "Half-Cock" work was when a SP-1 trade in
was resold, and the buyer brought it back due to jamming. Examination
revealed that there was an M-16 trigger and disconnector in the rifle...
The guy at the counter had missed it when he took the rifle in trade.
It took us 3 weeks to get the parts from Colt, and credited the guy with the money spent, and gave him another rifle.



Matt G
March 28, 2004, 11:58 AM
PM sent. Thanks, Tuner! :)

I personally see no problem with a little hole in the frame. The "liability" involved, to the administrators, is in fielding a real, live "mo-chine-gun", and putting it in the hands of those foolish officers. So long as the auto sears are in the safe, I doubt there's any problem.

March 28, 2004, 01:12 PM
As a bit of trivia...The grunts who were issued the M-14 in Vietnam and Germany will remembr this. I was able to carry an M-14 for a short time, and was later reissued the M-16...(which royally pissed me off!)

The M-14s were largely relegated to Semi-Auto status (even in a war zone) by removal of the selector lever and installation of the selector lock. This was by means small pin with a light interference fit, and pretty easy to change over by anyone with the right punch, and the lever and spring...
Neat way to conserve ammo and encourage fire discipline and aimed fire.



If you enjoyed reading about "The unthinkable: Converting M16A1's to semi-auto. Approx cost, involvement?" here in archive, you'll LOVE our community. Come join today for the full version!