This question popped up in a conversation at work today, and so far my Search-Fu has failed to produce any explanation.
Some weapons, like the Heckler & Koch MP5 submachine gun, has three different settings apart from safe mode - single, automatic and 3-shot burst.
My question - how does the 3-shot selection work mechanically in the gun? I’m assuming its some sort of rotating gizmo that spins around three times, but I was hoping somebody in the know could explain it to me.
I don’t know exactly how it works (I could tell you how the regular ones work), but here is a photo of the internal parts for an M16.
You can see that unlike a full-auto or semiauto M16/AR-15, there are two disconnectors (the flat, hooked bits that fit into the to of the trigger and hold the hammer back until the bolt has closed). There’s also a cam and another thing (the round thing), and the hammer, spring, and trigger are different.
Just to expand a bit: The disconnector at the top of the photo fits inside of the trigger. When the bolt (not pictured) runs back it cocks the hammer. In automatic mode (slelected by the selector, not pictured) the hook on the disconnector engages the notch on the back of the top of the hammer. When the bolt is in battery (locked, with a round in the chamber) the back of it hits the automatic sear (not pictured), which pivots the disconnector backward, which releases the hammer, which strikes the firing pin, which hits the primer and fires the round, starting the whole process over again.
As I said, I’ve never seen the three-shot burst assembly assembled. From the photograph it appears that the other disconnector goes on the outside of the trigger (note the offset spring). The hook on the front of the disconnector obviously engages the cam wheel, which appears to fit on the same pin as the hammer. How it works beyond that, I don’t know.
Cheers for the answer, I’ve studied the pictures and mulled over the details, but I must admit I’m still not sure how it works. But you’ve given me some ideas that’ll help further searching…
Looks like this is a brain teaser, and its hard to find relevant info on the web as well 
According to this patent, three-round burst is implemented by a rotating counter. To whit: “These objects are achieved by this invention which comprises a counter that is rotated by a reciprocating firing pin to count the rounds fired in the three round burst and then disconnects the trigger from the sear, which is thereby released to retain the firing pin, at the end of the burst.” There are also some pictures of a fully-assembled trigger assembly. I didn’t dig too deeply into the hundred or so labeled parts.
God, this is almost impossible to articulate without manipulating some visual aides so you can understand. But I’ll do my best.
Look at the picture Johnny linked to. Ignore the very top piece, cause he’s not going to come into play in burst mode. The selector lever pushes down on him so he doesn’t get involved.
Under that is a peice with two hooks. The front hook goes into that cam and the whole thing acts like a ratchet.
Notice the hammer. There is a notch at the very top, and a notch half-way up. We’ll call them notch 1 and notch 2 respectively. We’ll get back to those in a second.
Now look at the cam and that tiny spring. Whenever the hammer goes forward, the spring releases the cam and allows that front hook to keep it in place. As the hammer returns to the rear, the spring clutches the cam and causes it to rotate one notch, which is then grabbed by that front hook again.
When the three burst rounds are fired, the cam is ratcheting around, and notch 1 of the hammer is being caught by the auto sear (not pictured). The auto sear is basically just a flat piece of metal that catches notch 1 on the hammer. As the bolt goes forward, it pushes on the sear which releases the hammer.
After the third round is fired, the front hook is now in the next stop notch of the cam WHICH IS DEEPER THAN THE OTHERS. Since it is deeper, that whole connector piece with the two hooks is allowed to go forward just a little bit more, so now the rear hook catches notch 2 on the hammer. So the firing stops.
Once the trigger is released, the whole thing is reset and starts over.
This is a very simple system. One issue is that the cam will ratchet even in semi-auto mode which makes each trigger pull slightly different. At national matches, I’ve seen guys manually cycling their rifle three times between each shot so that they’re always squeezing the shortest trigger pull.
The other issue is that the system does not reset itself if the trigger is released before three rounds are fired. So if a soldier rapidly pulls and releases the trigger on burst, and only gets off two rounds, the next trigger pull will only fire ONE round. Not too big of a deal since soldiers dont really even use the burst anyway. The Army could easily issue everyone semi-autos and no one would notice.
HK uses a completely different system. There triggers will actually reset when you let your finger off, so a 3 round burst will always fire 3 rounds if you hold the trigger down. The internals of an HK trigger group are much more complicated, though. Lots of moving parts inside there.
Hope that helps.
That’s interestingly complex. That is not the current burst mechanism in an M16, though. The half dozen pieces that Johnny linked to are all that’s in there.
Thanks for the links to the patent Crescend - interesting read, and once I managed to make some sense of the illustrations I actually started understanding things.
And Bear Nenno, even without visuals your explanation was very good - Appreciate it! I can now go to work tomorrow thriumphantly explaining things. Always fun that
However, now I have to keep looking until i find some explanation of the HK mechanism as well, interesting to know how they do it. I have to stop by my friends house - he has a MP5 submachine gun, I’ll have to pick that appart and see if I can figure it out…
And yes, when I served in the Army I cant even remeber using anything but semi-auto mode - except a couple of times at the range when we were supposed to familiarise ourseves with the HK G3 7.62 rifle in full auto mode. Needless to say, while the first round might be on target the remaining 19 surely are not.
For the love of god, do NOT take the trigger group apart!! Look into it, and manipulate it (watch your knuckle), but do not dissasymble it. Unlike the half dozen pieces in an M16, the HK trigger groups are much more complicated and have many more parts. Just look here to satisfy your curiosity.
http://www.biggerhammer.net/manuals/HK_Trigger_Groups/
ETA: That’s a standard SEF group. The ones with 2 or 3 round burst are even more complicated.
Heheh duly noted - I wont fiddle with it.