How does 3 round burst fire work?


July 28, 2007, 08:01 PM
I can understand how full auto works, pretty much a sear without a disconnector in a sense (not exactly but you know what I mean)

But how in the hell does a mechanical trigger system work for 3 round burst? I've seen it in handguns like the VP70 and the berretta 93R and the M16A2 rifle. How is this accomplished, how does a trigger system know when exactly 3 rounds are fired? thats where I'm stumped.

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The Deer Hunter
July 28, 2007, 08:13 PM
I have often wondered the same thing. I don't know, but I kinda figure its like a round piece of metal that works kind of like a ratchet, and when it clicks three times it disconnects.

July 28, 2007, 08:17 PM
/\/\/\ You got it. There's a rotating cam in the trigger mechanism that engages the disconnecter after 3 shots. The problem with it is that if you release the trigger after only 2-shots, you're only going to get 1 shot the next time you pull the trigger rather than 3. IMHO, it only adds a lot of unnecessary complication (and parts that can break) to the M16 trigger group and is one of the worst ideas the US Army ever came up with.

July 28, 2007, 08:43 PM
Here's a picture showing the internals on a 3-round burst trigger-group; the little wheel with 6 teeth acts as the "counter" for a burst, with the wheel advancing one tooth every time the bolt carrier cycles, until it hits one of the large "stop" teeth.

Jim K
July 29, 2007, 01:48 PM
Just to clarify, on a gun like the M16, there definitely is a disconnector used in FA fire. Some folks think the hammer just follows down to fire the next shot, but that is a very unreliable (and dangerous) way to get repeated shots.

The sear disconnects, then after the bolt cycles and returns to battery, part of the bolt carrier or some similar part acts to trip the sear or an auxiliary sear to release the firing pin or hammer. In other words, the designer must make sure the bolt is fully locked before the next shot is fired.

Open bolt firing, with the forward-moving bolt acting as the firing pin, will work with pistol cartridges, but not with the more powerful rifle cartridges.


James T Thomas
July 30, 2007, 07:10 PM

I think just the opposite. I think it is one of the best ideas the Army has come up with.

They should adopt some form of it for the machineguns too; except for six or perhaps seven round bursts.

If you ever are able to enjoy someones' legal full auto weapon, try and see how many rounds you can keep within a human size and type target firing full automatic. Even at 10 m! Don't be deceived, after going off target, a full auto gun can be raked back across the target to chop it to pieces, but again, following three or more rounds, a correction in "pointing" has to be made.

Even the m.g; using sights as it it done, will buck of the group target being shot at after a half dozen rounds or so.

All it would take for you is to run out of ammo during the middle of a military battle, and you would change your mind.
This is the reason why so many Iraq posters here; infantry -have stated carrying several hundred or more rounds in the vehicle.

July 30, 2007, 07:12 PM
A related question: My friends m16 is pretty worn. It keyholes. I'm not sure when it was made. It has single shot and full if that helps. It fires fine on full but on single it will fire two shots pretty often. Is this just from a worn out sear?

July 30, 2007, 07:15 PM
James: It seems to me that proper trigger discipline can achieve the same results without relying on mechanical parts that can fail to work correctly or just fail to work at all.

From what I understand, most military units that are issued FA weapons are trained to fire short, controlled bursts in order to conserve their ammunition and increase hit rates. This works just as well while still allowing longer strings of fire when necessary.

July 30, 2007, 07:20 PM
“My friends m16 is pretty worn. It keyholes.”

Check and see how much rifling is left.

“Is this just from a worn out sear?”

Sounds like the hook that catches the sear (or the sear itself) for full auto is worn.
Going to be pricey to replace since an auto sear is treated the same as a full up gun.
Welding it up and repair might be a decent option.

July 30, 2007, 07:26 PM
My guess is the nub on the back of the hammer is worn.

And either the sear or the hammer should be cheap to replace provided you have the tax stamp for the M16.

Prolly your best bet is to take it to a smith who's paid the taxes to work on full auto stuff.

July 30, 2007, 07:34 PM
A m16a2 service rifle is 3 round-semi-safe only, the m16a4 is the same, the m4 is full-semi-safe. the m16a3 is a full auto. USMC went from the a2 to a4 because of that fact. Even on maching guns Marines are tought to fire in 3-5 round burst, that keeps the barrel from over heating and keeps rounds on target. Hope that helps

July 30, 2007, 08:25 PM
“My friends m16 is pretty worn. It keyholes.”

Also, what ammo are you using? An early M16 should have a 1:14 or 1:12 twist rate, which was marginal for the 55-grain FMJ, but won't be enough to stabilize a "heavy" bullet like the M855/SS109 style; the current rifles have a 1:7 twist.

July 30, 2007, 09:14 PM
As explained the 3 rd burst in a M16A2, M4 has a "memory". Short stroke the trigger, fire only 1 or 2 rds and the next time you get the 2 or 1 rd that's left on the cycle. This system has another bad point and when in 3 rd burst the trigger pull goes to 12 lbs or so.

I found I prefer the full auto option and usually fire 2 rd burst with a nice trigger, same as it is in semi.

H&K uses a different system which I can't explain how it works (I'd have to look it up). If you fire 1 or 2 rds with the H&K the next trigger pull will give you a full 3 rd burst.

I was a LE instructor and I would have some agents express disappointment in 3 rd burst. they figured it slowed you down if you had to dump a lot of rounds in a target. I'd take a MP5 with 3 rd or a full auto M4 and dump 30 rounds into the target in under 3 seconds. A M4 with 3 rd burst would take 4 seconds plus or minus due to the memory issue. Both the M4 and MP5 have cyclic rates of about 800 rpm and I'd be doing about 600 or 500 rds respectively. The group at 10 yds could be covered easily with your hand.

July 30, 2007, 10:37 PM
SDC- the ammo was wolf ammo, not sure what kind or even what kind are availible in wolf. I have a faint image of 55 grain fmj in red lettering on the bo x in my mind.

July 30, 2007, 11:14 PM
Ammo consumtion was another reason to go to 3 round burst

July 31, 2007, 02:07 AM
The three round burst limiter on the M-16A2 is a hardware solution to a sofware (training) problem. It is also a "kludgely" hardware solution as the burst mechanism has a detrimental effect on the trigger pull. Not only is the pull long and heavy, there are three distinctly different trigger pulls when firing semi-auto depending on whether you are firing the first, second, or third shot.

July 31, 2007, 02:11 AM

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