[Nalapombu]

Hey all,

Something I have always wondered about is the cyclic rate, 3 burst modes, and such on full auto rifles and handguns. I'd like to ask some questions and hope to get some information without the suspicion that I am out in my garage with a MAC10 trying to "Home Boy" it up.

Anyway, back to the REAL world. When he Thompson came out I read where the cyclic rate was too high, maybe like 900 to 1000 rounds per minute. It was taken and re tweaked and came out with a lower cyclic rate, Th same adjustments seem to happen with other main battle rifles before they are put into service. Happened with the M16 didn't it? It seems like it isn't that big of a problem to fix a cyclic rate that's too high. So I am curious to know what operations to tweak to get it to an acceptable number? Is it s process involving different spring tensions, or maybe letting less gas escape so the blowback wouldn't be as rapid and thus retard the cyclic rate for the arm.

Next is how the switch on the side is able to fire in single model, burst mode with 3 shots, Burst with 3 rounds and full auto. How does a lever like that able to control the different operating modes? One position you are shooting single shot, flip it to the next place and then you're at 3 round burst and the full auto setting that you can flip it to gives you a pure lead rainfall on your designated enemies. All by the flip of a simple switch.

How does that switch control that drastic difference in operation?

Can anyone explain to me how these things are accomplished in a full auto firearm? I wondered about it for years.

Thanks for your time.

Nalajr

 

[Trigger Finger]

Not much help, but I think you are mainly talking about factory settings! I think when the Army wanted the Thompson to shoot slower, the manufacturer increased the weight of the bolt, thus slowing it down to around 600-650 RPM.
The burst and full auto function by H&K is controlled by the entire trigger group I think.

 

[AK_Stick]

increase or decrease bolt weight.

increase or decrease buffer spring/buffer weight

increase or decrease size of gas ports

lengthen or shorten the placement of the gas port position on the barrel.

increase or decrease weight of op-rod or other associated operating parts.


All of those will change the effective ROF of a gun.

 

[M2 Carbine]

How does a lever like that able to comtrol cuh different operating modes?

The M2 Carbine is strickly mechanical.
This is a old Army film showing the M2's operation.
http://www.youtube.com/watch?v=o-J-uNcwnXo

 

[DFin]

I read somewhere that the significant engineering aspect in the design of the Grease Gun was the very slow rate of full auto fire. They used dual recoil springs and a heavy bolt.

 

[BassBait]

increase or decrease bolt weight.

increase or decrease buffer spring/buffer weight

increase or decrease size of gas ports

lengthen or shorten the placement of the gas port position on the barrel.

increase or decrease weight of op-rod or other associated operating parts.


All of those will change the effective ROF of a gun.

Plus One!

 

[Glockdude1]

increase or decrease bolt weight.

increase or decrease buffer spring/buffer weight

increase or decrease size of gas ports

lengthen or shorten the placement of the gas port position on the barrel.

increase or decrease weight of op-rod or other associated operating parts.


All of those will change the effective ROF of a gun.

:agree:

 

[DWARREN123]

Adjustable gas block or heavier bolt, size of gas hole/tubing and location for direct impringement types, cam locks, locking bolts and many more tricks.
The problem is not changing the rate but doing so in such a way that it remains reliable.

 

[GIockGuy24]

The amount of bolt travel distance is another way. The M-14 which was based on the Garand action had a tenth of an inch removed from the rear of the op rod where it strikes the front of the receiver. In the semi automatic Garand the bolt is suppose to strike the rear of the receiver at the same time the op rod strikes the front of the receiver. In the fully automatic M-14 this was found to cause the rate to be too fast for the magazine to feed cartridges. By removing the tenth of an inch from the op rod, the bolt strikes the receiver first, rebounding forward while the op rod is moving back to strike the receiver. This change in direction of the two recoiling parts slows the rate of fire. The Italian Beretta BM-59 is a select fire rifle based on the M1 Garand. It reduces the rate of fire through the use of numerous small levers and springs to slow the tripping of the hammer.

 

[mdfd]

increase or decrease bolt weight.

increase or decrease buffer spring/buffer weight

increase or decrease size of gas ports

lengthen or shorten the placement of the gas port position on the barrel.

increase or decrease weight of op-rod or other associated operating parts.


All of those will change the effective ROF of a gun.

+1 to this and also decreasing the "stroke" enough to shorten the time it takes for the bolt to travel back but enough to reliably strip the next round into the chamber...

 

[H&K 4 LIFE]

We use Blackjack buffers in the UZI to adjust the cyclic rate.

A 1.25" buffer is giving us almost 900rpm. with WWB.

 

[Glockdude1]

We use Blackjack buffers in the UZI to adjust the cyclic rate.

A 1.25" buffer is giving us almost 900rpm. with WWB.

Ever fired a Micro Uzi???

Very fast ROF!!

:supergrin:

 

[Berto]

I thought the thompson cyclic dropped when they got rid of the Blish delayed blowback and went to a heavier bolt and fixed fp.

 

[pesticidal]

We use Blackjack buffers in the UZI to adjust the cyclic rate.

A 1.25" buffer is giving us almost 900rpm. with WWB.

I found a way cheaper way to do that.

The washers are less than 50 cents ea.

 

[DJ Niner]

...

Next is how the switch on the side is able to fire in single model, burst mode with 3 shots, Burst with 3 rounds and full auto. How does a lever like that able to control the different operating modes? One position you are shooting single shot, flip it to the next place and then you're at 3 round burst and the full auto setting that you can flip it to gives you a pure lead rainfall on your designated enemies. All by the flip of a simple switch.

How does that switch control that drastic difference in operation?

Can anyone explain to me how these things are accomplished in a full auto firearm? I wondered about it for years.

Thanks for your time.

Nalajr

I'll take a shot at this one by attempting to describe the M16 versions. Bear with me, it's a bit complicated, and I'm fairly long-winded. :supergrin:

First, it's not just one or two parts that makes four modes of fire possible. You need a special hammer, hammer spring, burst cam, burst cam spring, two different disconnectors and springs (your normal semi-auto-only AR-15 only has one disconnector), safety/selector lever, trigger, and auto sear, along with a fully-automatic-capable receiver drilled for the auto sear pin and a full-auto bolt carrier to trip the auto sear in Auto and Burst modes.

When I refer to 4 modes of fire, the first mode is "Safe", which isn't really a firing mode; it's a non-firing mode. The second mode would be Semi, in which the weapon fires one shot for every pull of the trigger, and you must release the trigger to allow the mechanism to re-set to fire the next shot. The third mode would be Auto (short for automatic, commonly known as full-auto, so it's not confused with semi-auto mode). In Auto mode, the weapon will fire fully automatically until you release the trigger or the weapon runs out of ammunition (which happens surprisingly quickly -- around 2.5 seconds if you were loaded with a 30-shot magazine). The final mode is Burst, where the weapon will fire 3 shots in fully automatic mode, and you must release the trigger and press it again to fire another three shots. One quirk of the 3-shot mode is that the first time you move the selector to Burst and pull the trigger, you may not get 3 shots; you might only get one or two. Once the first 1/2/3-shot burst has been fired, the mechanism is properly prepped, and you'll get 3 shots each pull from then on (unless you release the trigger before all 3 are fired, in which case it will fire the remaining 1 or 2 shots with the next pull -- you could call it "finishing the last burst before starting the next", if you want).

So...

In Semi mode, when you pull the trigger, the weapon fires, and the semi-auto disconnector catches the hammer on the lower hammer hook, holding it to the rear until you release the trigger. Once the trigger is released, the hammer moves slightly forward and the nose of the trigger catches the sear notch at the bottom of the hammer, holding it fully cocked and waiting for the next pull on the trigger.

In Auto mode, when you pull the trigger, the weapon fires, and after the first shot the hammer is caught on the upper hammer hook by the auto sear. It holds it to the rear until the bolt carrier chambers and locks the next round into place, and as the bolt carrier moves the last one-eighth of an inch or so during closing, a flat cut-out surface on the bottom trips the auto sear, which releases the hammer to fire the next round. This continues until the trigger is released, where it would be caught and held to the rear by the trigger nose and hammer's sear notch, waiting for the next press to fire again.

In Burst mode, when you pull the trigger, the weapon fires, and after the first shot the hammer is caught on the upper hammer hook by the auto sear. It holds the hammer to the rear just like Auto mode, and when the bolt closes, the carrier trips the auto sear and releases the hammer to fire the next (2nd) shot, and the third. Once the third shot is fired, the Burst Cam (a small disk with notches cut out of it) cams the second (burst) disconnector upward to catch the lower hammer hook and hold the hammer to the rear, waiting for the trigger to be released (which would transfer the hammer to the trigger nose and sear notch), and another pull. The burst cam is advancing one notch with every shot, no matter if you are firing Semi, Auto, or Burst, and that's why you might get 1, 2, or 3 shots on the first pull after setting the selector to Burst; it all depends on where the burst cam is positioned in its 3-shot countdown.

The Safety/Selector lever makes it all possible by camming the two disconnectors and the auto sear in and out of engagement with the hammer in the various modes.

EDIT: I found a couple of animated .GIFs that show a cross-seciton of a semi-auto AR-15 firing, and an M16 firing in both Semi and Full modes. I hope the traffic doesn't annoy the other board, but this guy must have spent a LOT of time on these .GIFs, and they show it much better than I can describe it. Post #5:

http://www.xdtalk.com/forums/ar-talk/43376-how-tell-difference-between-ar-15-m16.html

Kudos to the talented person that set up those .GIFs! Bravo!


A look at the parts:

M16A2 burst parts:

The 4-mode parts would have a slightly different burst disconnector (dual-hook part) and safety/selector lever (not shown).


.

 

[Batesmotel]

I read somewhere that the significant engineering aspect in the design of the Grease Gun was the very slow rate of full auto fire. They used dual recoil springs and a heavy bolt.

Slow is an understatement. Super fun to fire.

 

[vikingsoftpaw]

With blow back and recoil operated weapons, the power of the ammo will make a difference in cyclic rate also. I was on a range while someone was videoing a S&W Mod 70 to demonstrate this. There was a 300 round per minute difference between Hot +P subgun ammo and the subsonics.

 

[DJ Niner]

With blow back and recoil operated weapons, the power of the ammo will make a difference in cyclic rate also. I was on a range while someone was videoing a S&W Mod 70 to demonstrate this. There was a 300 round per minute difference between Hot +P subgun ammo and the subsonics.

I've also seen this demonstrated with .22 rimfire machineguns. Load them with CCI Stingers (try not to think about the cost) and they cycle noticeably faster.

 

[samurairabbi]

Many auto-firing weapons are designed to use advanced primer ignition (API) to achieve a high cyclic rate. The firing pin is timed to strike the primer while the bolt is a tiny, yet noticable distance, from full closure with the breechface. The energy of the fired round is used to reverse the final motion of the bolt, rather than letting the breechface stop the bolt. This "floating bolt" effect can add greatly to the cyclic rate.

So if you want to SLOW the cyclic rate of such a firearm, you can simply reduce or eliminate this API use.

 

[PlasticGuy]

Lock time has an affect too. Using a heavier hammer increases the amount of time between tripping the sear and firing a round, which reduces rate of fire.