How does the 3-burst autosear work?

[BrokenPaw]

Discussion in another thread regarding the military's move from the full-auto M-16 to the 3-round burst version got me wondering...

How does the autosear in a burst-fire weapon know how many shots have been fired? What makes the third shot different from the first two, so that the sear doesn't release the hammer a fourth time?

I built my AR-15 lower half from a parts, so I'm familiar with the semi-auto sear and hammer and trigger interaction. I think I understand how a full-auto sear would work. It's the "stopping after 3" behavior that I'm not quite getting.

Can anyone explain this to a confused software weenie?

-BP

 

[SDC]

There's a 6-stepped wheel in the burst mechanism that "counts" the rounds as they're fired, but two of the steps (180 degrees apart) act as "stop" points. The way this operates in the A2 is that the wheel is released one notch for every shot that's fired, but it doesn't reset to zero unless you happen to get off the trigger at just the right time; ie. you could fire a 2-round burst, and then pull the trigger again expecting a 3-round burst, but you'll only get the last round from the previous burst.

 

[BrokenPaw]

Oooh! Cool! I was thinking that some sort of wheel would be required, but that seemed too...failure-prone, I suppose. Hmm. Thanks, SDC!

-BP

 

[Badger Arms]

I can't find the patent right now. I can direct you to the patent for the 4-position selector wich is nearly identical. Here:

http://www.biggerhammer.net/patents/US05760328__.pdf

 

[Kaylee]

am I imagining things, or does this mean if you're potentially dealing with several different trigger pulls in the same rifle?


-K

 

[Steve Smith]

You are correct, you'll get three different trigger pulls.

 

[voilsb]

somehow, though, I don't think trigger-pull is as big of a deal when in automatic fire as it is when trying to make aimed semi-automatic fire.

 

[Pinned&Recessed]

somehow, though, I don't think trigger-pull is as big of a deal when in automatic fire as it is when trying to make aimed semi-automatic fire.

Actually, there are still 3 different trigger pulls in semi-auto as well. The burst wheel is still rotating with each hammer fall and is affecting the trigger/hammer interaction. Assuming the wheel is at the start of the 3-round burst cycle, the trigger pull is heavy and gets lighter and crisper by the 3rd shot, then it gets heavy again.

What I want to know is how the HK 3-shot burst mechanism works. Even if you let off after 2 shots or end a mag on an odd number of rounds, it always resets itself to 3-shots. A spring perhaps?

The M16A1 with its Safe-Semi-Auto selector had a better, more consistent trigger pull.

The 3-round burst Fire Control Group also has a two-piece disconnector. When the weapon is on semi, the disconnector is in place like a regular AR or Auto M16.

In the burst gun, when he selector is moved to burst, only one of the disconnectors moves totally out of the way. When the tooth on the burst cam ratchets up, it engages the other disconnector, pushing it up allowing the hammer to get caught by the disconnector, not the auto-sear.

To make a 3-round burst rifle go Full-Auto, all you would have to do is remove the burst half of the disconnector and replace it with a washer of similar width. Then the burst cam has nothing to act on and the gun is a traditional Full-Auto.

Of course, the minute your unit armorer saw that modification, he would be crapping car parts, so I don't recommend it.

 

[Badger Arms]

The difference in pull weight was about a pound or two different gun to gun average. The Marines don't like this.

 

[Hkmp5sd]

The Marines don't like this.

They are not the only one!

 

[voilsb]

Actually, there are still 3 different trigger pulls in semi-auto as well. The burst wheel is still rotating with each hammer fall and is affecting the trigger/hammer interaction. Assuming the wheel is at the start of the 3-round burst cycle, the trigger pull is heavy and gets lighter and crisper by the 3rd shot, then it gets heavy again.

didn't know this. consider my comment hereby retracted.

 

[Hkmp5sd]

I like what Heckler & Koch: Armorers of the Free World has to say about the M16A2 3-shot burst....

At least, Heckler & Koch's mechanism doesn't compromise the quality of the trigger pull anything like the dreadful three-shot burst control device found on the U.S. M16A2 rifle.

Unfortunately, it doesn't say how they go about doing it.

 

[Badger Arms]

Here's a quote from Stevens' "The Black Rifle" on the subject:

 

[Pinned&Recessed]

OK. That article corrected me. My mind was fuzzy and I had it bass-ackwards. The trigger pull starts light and gets progressively heavier and muddier.

First thing to go is the mind and all that.