M4/M16 full auto trigger , How does it work

Metal god · February 27, 2015, 03:31 PM

I've been thinking about this and I was wondering how they work in detail . I know how to put a standard semi auto together to get full auto but really all your doing is letting the rifle slam fire . How does the M4/M16 have such a high rate of fire and not be slam firing . Is the bolt really locking up completely before the hammer is let go ? Is it some sort of timing thing that holds the hammer just a fraction of a second so it does not ride the BCG home ?

Kaylee · February 27, 2015, 04:14 PM

https://www.youtube.com/watch?v=NtwhZj1_TlI
(auto stuff starts around 16 minutes in)

rickyrick · February 27, 2015, 04:14 PM

Yes, but I can't explain it well enough...

Machineguntony · February 27, 2015, 04:25 PM

The M16 and the Ar15 does not slam fire; and yes, they lock up completely before the next round is discharged, in full auto. Both, if put into full auto mode, work similarly, but somewhat differently. I have both M16s and Ar15s that are full auto, and the RDIAS is so simple that it is ingenious.

The difference between the M16 and the Ar15 is that the M16 uses a GI sear, whereas the Ar15 uses either a lightening link or a RDIAS. Both AR and M16 require M16 parts that depress the disconnector. Then, once the disconnector is depressed, the sear catches the hammer (which needs to have a full auto notch), and then the bolt trips the sear (aka the catch), and thereby releasing the hammer once again.

The reason the M16/Ar15 isn't a slam fire gun is because technically, the hammer is being caught with each cycle by the GIsear/DIAS and then, the hammer is released by the forward traveling bolt, which trips the sear.

Edit: [the M60 does not slam fire].

The way I think of it is that in an M16/AR15, the bolt, action, and hammer are cycled and locked completely, as if they were semi auto guns, but in a full auto gun, the GIsear/RDIAS is pulling the trigger for you.

marine6680 · February 27, 2015, 06:28 PM

Full auto has an auto sear, that holds onto an extra hook on the hammer... when the bolt is fully into battery, the auto sear is tripped to release the hammer...

Think back to AR profile bolt carriers, and M16 (full auto) profile bolt carriers.

The AR carrier has less material at the lower rear section... This is because that area on a full auto carrier is what allows the auto sear to work.

The auto sear sits at the rear of the lower receiver, and when the carrier moves forward to the point that the bolt is closed; the lower portion of the carrier comes into contact with the auto sear, and trips it to let the hammer fall.

If you let go of the trigger, the auto sear is still tripped, but the semi auto sear is in place to catch the hammer after it falls a very small amount when the auto sear catches it.

I actually find the burst trigger more interesting, as it is more complex.

Metal god · February 27, 2015, 09:17 PM

Very cool guys thanks for the explanation and video .

Now feel free to explian the three round burst

James K · February 27, 2015, 09:55 PM

The full auto version of the AR-15 works the same as the M16. The DIAS and lightning link are after market devices intended to provide a full auto capability to semi-auto AR-15's. Both require use of some factory full auto parts, and both are illegal unless registered, as is any rifle made as semi-auto and converted to full auto or selective fire.

The full auto system has two parts. The first is a selector that works in conjunction with a special hammer to keep the normal (bottom) sear from engaging the hammer when set for full auto fire. In other words, when the bolt closes, the hammer will begin to follow it down. But it can't because the full auto sear catches the hook on the top of the hammer and holds it back until the bolt carrier moving forward strikes the top end of the auto sear, pivoting it forward. That moves the bottom end of the auto sear back, out of engagement with the hammer, allowing it to fall.

Since this takes place on the final forward movement of the carrier, after the bolt is closed and fully locked, the gun cannot fire unless the bolt is locked. The M16 is not "slamfire", nor does it depend on firing pin inertia. Those who think that simply don't know how auto weapons work.

Incidentally, the M60 is not "slamfire." Like other auto weapons (except low power SMGs like the M3A1) the bolt is fully locked before the firing pin is released.

Jim

Machineguntony · February 27, 2015, 10:18 PM

James, what locks the bolt on an M60 back? I always thought that the job of the sear catch was to lock the bolt back. On the M60 the sear just stays out of the way while the gun is firing.

**44 AMP** · February 28, 2015, 12:06 AM

Quote:

An example of a gun that is designed to slam fire is an M60.

No, it only looks like that. The bolt is rotated shut (locked) before the firing pin hits the primer.

Quote:

what locks the bolt on an M60 back? I always thought that the job of the sear catch was to lock the bolt back. On the M60 the sear just stays out of the way while the gun is firing.

It's the sear. And, technically the sear has to stay out of the way in order to fire any of our guns.

Between the M16 and the M60, we have examples of the three common methods of firing. Closed bolt semi auto, closed bolt full auto (M16) and open bolt full auto (M60). Years ago you could get an open bolt semi auto, but the ATF has essentially driven those off the market, and they were about useless, anyway.

When you fire the M16 semi auto, the sear (part of the trigger in this gun) releases the hammer, firing the round. Bolt carrier comes back, ejecting the fired case, pushing the hammer down, then starts forward, feeding the next round. As it moves forward, the hammer starts to follow it. The hammer is then caught and held by the disconnector. You are still holding the trigger back, at this point. When you release the trigger, the disconnector lets go of the hammer, which is then caught and held by the sear (nose of the trigger). That's the little click you hear when you let go of the trigger, the mechanism "resetting" for another semi auto shot.

When you switch the M16 to full auto, the selector lever holds the disconnector away from the hammer, and the hammer is instead caught by the auto sear (a different part, not the trigger). The auto sear holds a hook on the GI hammer until it is tripped by the bolt carrier during the last part of its forward movement after the bolt has locked shut. Since you are still holding the trigger back, the trigger sear is not there to catch the hammer, and it falls all the way, firing the round and starting the cycle over. This continues until you release the trigger, then, after you do, the next time the hammer is tripped off the auto sear, the trigger sear catches and holds it.

The entire firing cycle both semi and full auto start and end with the bolt closed.

The M60 is the opposite. Its firing cycle starts and ends with an open bolt.

In the M60, you pull the bolt back (op rod, actually), and it is caught and held back by the sear. The sear in an M60 is a separate part, operated by the trigger. Pulling the trigger pulls the sear down, releasing the op rod, when runs forward causing the bolt to chamber a round, lock, and fire (in that order). The gas system drives the op rod back, ejecting the fired case, and the op rod spring sends it forward again repeating the cycle until you let go of the trigger. When the trigger is released, the sear rises, and catches the op rod on its next trip back, and holds it open.

hope this helps

Machineguntony · February 28, 2015, 12:54 AM

Ah yes. Thanks, 44 for that description. I took apart my m60 and tried to figure it out. I assumed that since the sear operated differently from the M16, that the bolt was simply slamming back and forth. Also, with the trigger operating the sear, it did seem that way.

I figured there was a reason why the bolt turned.

Also, why don't we have an AR/M16 forum, or at least add to this section of the forum. There isn't enough discussion of the beautiful M16/AR, as there isn't too much interest in the NFA section.

marine6680 · March 1, 2015, 01:04 AM

Burst mode uses a second "burst" disconnector that follow a ratcheting cam. Once the cam rotates through the three round burst, the burst disconnector is allowed to engage the hammer and prevent the hammer from falling again.

I found an animated video that explains all three types.

Here it is

There are 4 way selectors available too, that allow semi, burst , and auto.

In semi mode, the semi disconnector does its job.

In burst, the semi disconnector is disabled and the burst disconnector is working.

In auto the burst disconnector is disabled as well, and only the auto sear is engaged.

skizzums · March 1, 2015, 01:43 AM

thanks for that youtube link. I have never seen it explained so clearly. very simple.

James K · March 4, 2015, 12:28 AM

I am glad to see that. I had no desire at all to get into explaining the burst mode.

Open bolt firing is used on most machineguns to allow the gun to cool better during interruptions in firing and prevent a round from "cooking off" in a hot chamber.

But open bolt firing in submachine guns has other purposes, to simplify the gun and reduce bolt mass. The latter is done by something called "advanced primer ignition." The bolt usually has the firing pin made as a "tit" on the bolt face, though some guns have a separate firing pin. When the bolt pushes a round out of the magazine and chambers it, the round stops on the chamber shoulder.

But the bolt is NOT QUITE fully closed when the firing pin hits the primer. The primer fires, the powder ignites and the pressure builds, all while the bolt still has forward momentum. That means the pressure trying to blow the case back out of the chamber has to evercome not only the mass of the bolt, but also its remaining forward momentum, making it act heavier than it is. So the bolt mass and recoil spring can be reduced.

When a SMG is converted to closed bolt firing, API does not work, so the bolt must be more massive and the recoil spring has to be heavier to slow its backward force and prevent damage to the gun. A good example is the semi-auto Thompson, which requires a lot of strength to cock, while the original gun can be cocked with the little finger.

Jim

**44 AMP** · March 5, 2015, 03:46 AM

Not only does API not work, but you are also compressing a firing pin spring that the SMG does not have.

According to gun myth & legend, the internal design of the semi auto Thompson was deliberate done..oddly... to make it difficult to convert to full auto. It is less than the best possible design, done so that the ATF would approve it for sale.

A more "straight forward" conversion of the design from full auto to semi auto is possible, and would result in a gun that doesn't take three men, a boy, and a gorilla to cock. However, if the ATF won't approve it, you can't sell it, so you make what you know you can sell.

James K · March 5, 2015, 05:13 PM

The original Auto Ordnance Corp. made a semi-auto "carbine" called the Model 1927. They were simply converted SMG's and used the same bolt and firing system except that the disconnector was fixed in the upward position instead of being moved up and down by the selector lever. It fired from an open bolt.

BATFE won't allow open-bolt semi-autos to be sold since it contends (correctly) that they are too easy to convert to automatic fire and thus are machineguns under the definition in the law.

Jim

**44 AMP** · March 6, 2015, 04:31 AM

I have one of the Auto Ordnance (Numrich) 1927A1s. The internals are quite different from the SMG or the original open bolt 1927 semi.

The bolt (and other parts) are 1/4" "off" from the SMG size, and full auto parts won't fit in the gun. Standard Tommygun magazines (commercial or GI) must be modified to work in the gun. Removing the drum takes 3 hands. (the drum comes with a special tool called a "3rd hand")

Mine came with a manual from Kahr, but the gun doesn't say Kahr anywhere on it, which leads me to think it was built (but not shipped) before Kahr took them over.

James K · March 6, 2015, 11:07 PM

I haven't seen one for a while, so I don't know what they are marking them now, but Numrich (Gun Parts Corp.) bought the Auto Ordnance name and marked their guns the same way as the old ones except with the West Hurley address. When Kahr bought GPC, they got the Auto Ordnance name also.

The changes to closed bolt firing and to the parts and magazines were forced on them by BATFE; IIRC, the original Model 1927, (should you be lucky enough to own one) is classified as a machinegun and has to be registered as such. The lack of API forced the use of a heavy bolt and heavy spring, so cocking them requires two men and three small boys.

Jim

February 27, 2015, 03:31 PM	#1
Metal god Senior Member Join Date: April 10, 2012 Location: San Diego CA Posts: 7,126	M4/M16 full auto trigger , How does it work I've been thinking about this and I was wondering how they work in detail . I know how to put a standard semi auto together to get full auto but really all your doing is letting the rifle slam fire . How does the M4/M16 have such a high rate of fire and not be slam firing . Is the bolt really locking up completely before the hammer is let go ? Is it some sort of timing thing that holds the hammer just a fraction of a second so it does not ride the BCG home ? __________________ If Jesus had a gun , he'd probably still be alive ! I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again .

February 27, 2015, 04:14 PM	#3
rickyrick Senior Member Join Date: March 15, 2010 Posts: 8,386	Yes, but I can't explain it well enough... __________________ Woohoo, I’m back In Texas!!!

February 27, 2015, 04:25 PM	#4
Machineguntony Senior Member Join Date: June 22, 2013 Posts: 1,277	The M16 and the Ar15 does not slam fire; and yes, they lock up completely before the next round is discharged, in full auto. Both, if put into full auto mode, work similarly, but somewhat differently. I have both M16s and Ar15s that are full auto, and the RDIAS is so simple that it is ingenious. The difference between the M16 and the Ar15 is that the M16 uses a GI sear, whereas the Ar15 uses either a lightening link or a RDIAS. Both AR and M16 require M16 parts that depress the disconnector. Then, once the disconnector is depressed, the sear catches the hammer (which needs to have a full auto notch), and then the bolt trips the sear (aka the catch), and thereby releasing the hammer once again. The reason the M16/Ar15 isn't a slam fire gun is because technically, the hammer is being caught with each cycle by the GIsear/DIAS and then, the hammer is released by the forward traveling bolt, which trips the sear. Edit: [the M60 does not slam fire]. The way I think of it is that in an M16/AR15, the bolt, action, and hammer are cycled and locked completely, as if they were semi auto guns, but in a full auto gun, the GIsear/RDIAS is pulling the trigger for you. Last edited by Machineguntony; February 28, 2015 at 12:59 AM.

February 27, 2015, 09:17 PM	#6
Metal god Senior Member Join Date: April 10, 2012 Location: San Diego CA Posts: 7,126	Very cool guys thanks for the explanation and video . Now feel free to explian the three round burst __________________ If Jesus had a gun , he'd probably still be alive ! I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again .

February 27, 2015, 09:55 PM	#7
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	The full auto version of the AR-15 works the same as the M16. The DIAS and lightning link are after market devices intended to provide a full auto capability to semi-auto AR-15's. Both require use of some factory full auto parts, and both are illegal unless registered, as is any rifle made as semi-auto and converted to full auto or selective fire. The full auto system has two parts. The first is a selector that works in conjunction with a special hammer to keep the normal (bottom) sear from engaging the hammer when set for full auto fire. In other words, when the bolt closes, the hammer will begin to follow it down. But it can't because the full auto sear catches the hook on the top of the hammer and holds it back until the bolt carrier moving forward strikes the top end of the auto sear, pivoting it forward. That moves the bottom end of the auto sear back, out of engagement with the hammer, allowing it to fall. Since this takes place on the final forward movement of the carrier, after the bolt is closed and fully locked, the gun cannot fire unless the bolt is locked. The M16 is not "slamfire", nor does it depend on firing pin inertia. Those who think that simply don't know how auto weapons work. Incidentally, the M60 is not "slamfire." Like other auto weapons (except low power SMGs like the M3A1) the bolt is fully locked before the firing pin is released. Jim Last edited by James K; February 27, 2015 at 10:03 PM.

February 27, 2015, 04:14 PM	#2
Kaylee Senior Member Join Date: June 15, 2000 Location: The Last Homely House Posts: 1,677	https://www.youtube.com/watch?v=NtwhZj1_TlI (auto stuff starts around 16 minutes in)

February 27, 2015, 06:28 PM	#5
marine6680 Senior Member Join Date: July 24, 2012 Location: Parker, CO Posts: 4,594	Full auto has an auto sear, that holds onto an extra hook on the hammer... when the bolt is fully into battery, the auto sear is tripped to release the hammer... Think back to AR profile bolt carriers, and M16 (full auto) profile bolt carriers. The AR carrier has less material at the lower rear section... This is because that area on a full auto carrier is what allows the auto sear to work. The auto sear sits at the rear of the lower receiver, and when the carrier moves forward to the point that the bolt is closed; the lower portion of the carrier comes into contact with the auto sear, and trips it to let the hammer fall. If you let go of the trigger, the auto sear is still tripped, but the semi auto sear is in place to catch the hammer after it falls a very small amount when the auto sear catches it. I actually find the burst trigger more interesting, as it is more complex.

February 27, 2015, 10:18 PM	#8
Machineguntony Senior Member Join Date: June 22, 2013 Posts: 1,277	James, what locks the bolt on an M60 back? I always thought that the job of the sear catch was to lock the bolt back. On the M60 the sear just stays out of the way while the gun is firing.

February 28, 2015, 12:54 AM	#10
Machineguntony Senior Member Join Date: June 22, 2013 Posts: 1,277	Ah yes. Thanks, 44 for that description. I took apart my m60 and tried to figure it out. I assumed that since the sear operated differently from the M16, that the bolt was simply slamming back and forth. Also, with the trigger operating the sear, it did seem that way. I figured there was a reason why the bolt turned. Also, why don't we have an AR/M16 forum, or at least add to this section of the forum. There isn't enough discussion of the beautiful M16/AR, as there isn't too much interest in the NFA section.

March 1, 2015, 01:04 AM	#11
marine6680 Senior Member Join Date: July 24, 2012 Location: Parker, CO Posts: 4,594	Burst mode uses a second "burst" disconnector that follow a ratcheting cam. Once the cam rotates through the three round burst, the burst disconnector is allowed to engage the hammer and prevent the hammer from falling again. I found an animated video that explains all three types. Here it is There are 4 way selectors available too, that allow semi, burst , and auto. In semi mode, the semi disconnector does its job. In burst, the semi disconnector is disabled and the burst disconnector is working. In auto the burst disconnector is disabled as well, and only the auto sear is engaged.

March 1, 2015, 01:43 AM	#12
skizzums Senior Member Join Date: July 1, 2013 Location: Douglasville, Ga Posts: 4,615	thanks for that youtube link. I have never seen it explained so clearly. very simple.

March 4, 2015, 12:28 AM	#13
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	I am glad to see that. I had no desire at all to get into explaining the burst mode. Open bolt firing is used on most machineguns to allow the gun to cool better during interruptions in firing and prevent a round from "cooking off" in a hot chamber. But open bolt firing in submachine guns has other purposes, to simplify the gun and reduce bolt mass. The latter is done by something called "advanced primer ignition." The bolt usually has the firing pin made as a "tit" on the bolt face, though some guns have a separate firing pin. When the bolt pushes a round out of the magazine and chambers it, the round stops on the chamber shoulder. But the bolt is NOT QUITE fully closed when the firing pin hits the primer. The primer fires, the powder ignites and the pressure builds, all while the bolt still has forward momentum. That means the pressure trying to blow the case back out of the chamber has to evercome not only the mass of the bolt, but also its remaining forward momentum, making it act heavier than it is. So the bolt mass and recoil spring can be reduced. When a SMG is converted to closed bolt firing, API does not work, so the bolt must be more massive and the recoil spring has to be heavier to slow its backward force and prevent damage to the gun. A good example is the semi-auto Thompson, which requires a lot of strength to cock, while the original gun can be cocked with the little finger. Jim

March 5, 2015, 03:46 AM	#14
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 30,460	Not only does API not work, but you are also compressing a firing pin spring that the SMG does not have. According to gun myth & legend, the internal design of the semi auto Thompson was deliberate done..oddly... to make it difficult to convert to full auto. It is less than the best possible design, done so that the ATF would approve it for sale. A more "straight forward" conversion of the design from full auto to semi auto is possible, and would result in a gun that doesn't take three men, a boy, and a gorilla to cock. However, if the ATF won't approve it, you can't sell it, so you make what you know you can sell. __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

March 5, 2015, 05:13 PM	#15
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	The original Auto Ordnance Corp. made a semi-auto "carbine" called the Model 1927. They were simply converted SMG's and used the same bolt and firing system except that the disconnector was fixed in the upward position instead of being moved up and down by the selector lever. It fired from an open bolt. BATFE won't allow open-bolt semi-autos to be sold since it contends (correctly) that they are too easy to convert to automatic fire and thus are machineguns under the definition in the law. Jim

March 6, 2015, 04:31 AM	#16
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 30,460	I have one of the Auto Ordnance (Numrich) 1927A1s. The internals are quite different from the SMG or the original open bolt 1927 semi. The bolt (and other parts) are 1/4" "off" from the SMG size, and full auto parts won't fit in the gun. Standard Tommygun magazines (commercial or GI) must be modified to work in the gun. Removing the drum takes 3 hands. (the drum comes with a special tool called a "3rd hand") Mine came with a manual from Kahr, but the gun doesn't say Kahr anywhere on it, which leads me to think it was built (but not shipped) before Kahr took them over. __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

March 6, 2015, 11:07 PM	#17
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	I haven't seen one for a while, so I don't know what they are marking them now, but Numrich (Gun Parts Corp.) bought the Auto Ordnance name and marked their guns the same way as the old ones except with the West Hurley address. When Kahr bought GPC, they got the Auto Ordnance name also. The changes to closed bolt firing and to the parts and magazines were forced on them by BATFE; IIRC, the original Model 1927, (should you be lucky enough to own one) is classified as a machinegun and has to be registered as such. The lack of API forced the use of a heavy bolt and heavy spring, so cocking them requires two men and three small boys. Jim