The engineering and design behind burst fire


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A.M. Baer
November 21, 2008, 04:17 AM
I know how semi-automatic firearms work, and have a general knowledge of fully-automatic firearms, but I cannot for the life of me figure out how burst fire works. I have looked all over the interweb and thought it through on my own, but still cannot come up with any ideas. My top two ideas of how it works is either 1, magic or 2, a computer chip.

Does anyone have an explanation or diagram showing how burst fire firearms work? (Just to note, this is out of curiosity and a desire for knowledge; I am not seeking information to convert any firearms illegally).

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Deus Machina
November 21, 2008, 04:27 AM
I always thought it was a sort of ratchet system, that would move a piece with each shot until it caught, or released the catch.

But I've never seen it.

Now you've got me curious, too, durnit.

Highland Ranger
November 21, 2008, 06:22 AM
Ratcheting cam . . . .

http://www.antipersonnel.net/links/R006.html

http://en.wikipedia.org/wiki/Burst_mode_(firearm)

http://www.freepatentsonline.com/4594934.html

taliv
November 21, 2008, 08:46 AM
anybody remember the old days where the first ten posts after a question like that would be variations on the "you can't talk about that" theme and accusing the OP of being an undercover ATF agent?

wonder if those days are coming again...

Highland Ranger
November 21, 2008, 08:59 AM
Didn't even think of it . . . new member, information available on first three google hits, "interweb" . . . . just struck me as a novice user not Agent Schmuckatelli.

What law would be violated anyways?

rbernie
November 21, 2008, 09:25 AM
anybody remember the old days where the first ten posts after a question like that would be variations on the "you can't talk about that" theme and accusing the OP of being an undercover ATF agent?Yes, I do - but you've miscast the issue a bit.

The concern (and thread lock) usually happened when somebody would post the equivalent of 'If I wanted to, not that I would, what would I do to an AR15 to make it FA?'.

There is a difference between asking for FCG design clarification and (seemingly) asking for physical modification advice. Both may have wanted to get to the same place (knowledge only about firearms design), but one asked that clearly and the other asked it in a way that could be misconstrued as having bad intent.

And that difference defines what it means to be THR.

Z71
November 21, 2008, 10:23 AM
If I recall, it is a ratcheting doohicky/autosear thingybob if your talking about M16A2 rifles.

Been a long time since I feild stripped an M16 A2, however I don't recall that the A2 had many more parts than the M16A1. The trigger group parts just looked a bit different between the two.

Shear_stress
November 21, 2008, 10:29 AM
This should be of some use:

http://www.ar15.com/content/guides/function/burst/

eldon519
November 21, 2008, 10:42 AM
Solid question. I've wondered about that many a times and never bothered to ask.

blkbrd666
November 21, 2008, 10:55 AM
Yep, it's the "ratcheting doohicky/autosear thingybob", and the cam is the key. Shear_stress's link is a good one!

xd45gaper
November 21, 2008, 11:05 AM
no you guys are all wrong its magic, ive thunked this along time now and i know most rifles with burst capability dont have a micro chip so it has to be freaking magic!

Still 2 Many Choices!?
November 21, 2008, 11:36 AM
Its actually pretty slick. Two disconnectors are used. One semi auto( that gets cammed/pushed up out of the way by the M-16 selector when swithced to "burst"), and a second "burst limiting disconnector" with the ratchet. This one does not get moved by the selector switch to burst mode. In "rock and roll" mode,(or is it rythm and blues with 3rnd burst LOL ), the second ratcheted burst limiting disconnector, has three teethe cut into the ratchet. One is much deeper than the other two. The deep cut tooth on the ratchet allows the burst limiting dissconector to move forward more than the other two teethe. This extra travel allows the burst limiter to catch the hammer after the third shot. Requiring the shooter to reset the trigger from the burst limiting disconnector back to the primary sear engagement( trigger's face).

PS- On the first shot after switching to "burst mode" you might get one, two, or three shots depending on which toothe the burst limiter was on when burst firing commenced. It also makes the trigger pull retarded( or change) from shot to shot. From what I've read on the web:uhoh: , the H&K burst set up does not have the two problems I mention above. Ie... You always get a 3 round burst, when you switch firing modes, and the trigger pull stays consistent... HTH.

Still 2 Many Choices!?

briansmithwins
November 21, 2008, 01:42 PM
Diagrams from the TM.

BSW

http://i2.photobucket.com/albums/y3/briansmithwins/fafunction1.jpg

http://i2.photobucket.com/albums/y3/briansmithwins/fafunction2.jpg

http://i2.photobucket.com/albums/y3/briansmithwins/fafunction3.jpg

http://i2.photobucket.com/albums/y3/briansmithwins/fafunction4.jpg

http://i2.photobucket.com/albums/y3/briansmithwins/fafunction5.jpg

LGswift
November 21, 2008, 04:11 PM
I had a friend tell me that some other country had an ar 15 with a 4 position selector with the burst and automatic. In the CF our c7s (m16a1e1's) are automatic only and based on the parts used for the ratcheting 3 round burst, I told him that it was not possible to have 3rnd burst and automatic in the FCG of an ar15.

Can anyone confirm or deny this?

CAVHOOAH
November 21, 2008, 04:23 PM
it is possible....check out gunbroker.

DoubleTapDrew
November 21, 2008, 04:28 PM
There was a company (possibly Tapco?) that sold 4 position selector set-ups to make your M-16 do both burst and FA (it noted for M-16 owners only NFA rules apply, blah blah). I think it was about $150. Don't know if it was ever a factory option because IIRC it's m16a1 & a3=full auto, a2,a4=burst

A.M. Baer
November 21, 2008, 06:02 PM
Well thank you very much for the information; certainly answered my questions. Makes me want to go through all the BATFE/NFA stuff to get one.

Shear_stress
November 21, 2008, 06:32 PM
The AR design is clever as hell, but has the slight disadvantage of not resetting if you let up on the trigger mid-burst. Some other designs (HK?) give you the same burst on the first trigger pull every time.

taliv
November 21, 2008, 07:57 PM
LGswift, if by "some other country", you mean "the US", then yeah, it's not only possible, I've handled 2 of them this year. quite rare, but they're out there

Still 2 Many Choices!?
November 21, 2008, 09:41 PM
Thanx for the diagram, it explains things exponentially better than I did LOL :o. If I had my way, I would issue the M-16, and the M4 with single shot, two round burst a.k.a. a" hammer"(not 3rnd burst), and auto trigger group. I would rather have 15 trigger pulls per mag than 10, but I guess that's just me.

Still 2 Many Choices!?

nicholst55
November 21, 2008, 11:25 PM
IIRC, DPMS and Bushmaster both sell 4-position FCGs.

Personally, I loathe the 3-rd burst setup on the M16A2 for all the reasons cited above. That, and all that crap wears out 3 times faster than the M16A1 full auto parts do.

If ever there was a solution in search of a problem, IMHO the 3-rd burst mechanism is it!

LGswift
November 22, 2008, 11:46 AM
LGswift, if by "some other country", you mean "the US", then yeah, it's not only possible, I've handled 2 of them this year. quite rare, but they're out there

By some other country i meant outside of north america. I figured if it were possible to manufacture fcg parts like this they would be more common, but as you say they are rare indeed as i have never heard of it confirmed until now.

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