View Full Version : Another AR nooby question

Big Caliber
January 20, 2013, 08:56 PM
I'm really new to center fire AR's, so forgive me for asking simpleton questions in a piece meal fashion. The more I "play" with it, the more questions I come up with. I'm just trying to better understand the "whys" of the design of my new rifle. Why isn't the front sight located closer to the muzzle, say right behind the muzzle brake? The barrel is only 16" long. And this is a civilian model. Or is it just another case of easier production to copy the military model?

January 20, 2013, 09:01 PM
I may be wrong here? I was!

January 20, 2013, 09:34 PM
The AR-15 works on Stoner's direct gas Impingement system. When the bullet passes the gas port in the barrel the gas it tapped off to the bolt carrier and builds pressure and starts the rearward movement of the bolt carrier. The reason the sight (and the gas manifold) is a few inches back of the muzzle is that it takes that amount of "dwell time" to build the correct gas pressure before the bullet exits the muzzles so the bolt carrier will be moving at the correct speed to function the action. Too little dwell time for gas to build results in short cycles.

January 20, 2013, 09:38 PM
Original AR's had a longer barrel and hence, a longer sight radius. When they went to the carbine version the same sight/gas block was kept for ease of manufacture.

As for being a short range weapon, They are reasonably accurate out to 500 meters. For all practical purposes, that is a long distance for a combat situation.

I have shot my AR15-A2 with a standard length 20" match barrel quite accurately out to 600 yards.

January 20, 2013, 09:43 PM
See I knew I should have kept my mouth shut! LOL I was not even close on that one!:o

January 20, 2013, 09:47 PM
Wyosmith, if that's the case, then how do you explain dissipator models that move the gas block forward on a 16" barrel?

January 20, 2013, 10:09 PM
The original dissipator to my knowledge used a carbine length gas system hidden under the rifle length handguard and an FSB with a fake gas block mounted out front.

Big Caliber
January 21, 2013, 12:10 AM
So...the front sight is part of the gas system then?

January 21, 2013, 12:24 AM
So...the front sight is part of the gas system then?

Yep. the gas tube takes gas from the barrel and directs it to the bolt.

The idea behind the dissapator is to use a carbine length barrel with a rifle length gas system. That way the gas is at a lower pressure when it is tapped off of the barrel as it is further from the chamber. The idea is that it doesn't cause the action to work as hard, that is, the gas impulse is not a strong as when it is tapped off closer. There is some evidence that there is less wear with this set up.

Metal god
January 21, 2013, 03:53 AM
That way the gas is at a lower pressure when it is tapped off of the barrel as it is further from the chamber.

I'm not sure this is correct . They way I understand it is , it's all about the dwell time . It does not matter as much how far the gas block is away from the chamber as how much barrel there is after the gas block . Think of it like this .The bullet is plugging the barrel as it goes down the bore and as long as the bullet is in the barrel there is a lot of pressure in there . As the bullet passes the gas block the gas and pressure gets redirected down the gas tube . A carbine length gas system is 7-1/2" leaving 8-1/2" of dwell time in a 16" barrel. If you were to cut that barrel down to just in front of that gas block . The pressure would drop off immediately as the bullet left the barrel and there would not be enough pressure to operate the bolt . The opposite is true as well . If you had a 7-1/2" gas system on a 20" barrel there would be to much pressure , for to long of a time as the bullet plugged the barrel for that 12-1/2 " past the gas block . I believe the original M16 had a 20" barrel with a 12" gas system leaving that 8" of dwell time . Now there are all kinds of ways to play with that system from spring stiffness to buffer weights and the size of the gas port but generally speaking it's all about the dwell time .

January 21, 2013, 08:27 AM
Hmmm..... Hidden gas block.... Sneaky sneaky

I'll have to check one out the next time I see one. I knew that the dissipator gave you rifle-length sight radius, which was the big selling point over the shorter sight radius of the carbines, but seeing that FSB out there made me think they went with a mid-length gas system to reduce the gas impulse that is oft-complained about in the carbine length platforms. The dwell time theory make sense, however.

January 21, 2013, 09:01 AM
There are different barrel configurations without the gas-block/front sight. Some people choose to use a rail system for mounting a sight. An AR doesn't have to come in a configuration with the gas-block/front sight.

The gas-block combined with front sight is just an efficient, light-weight design that I guess the military likes.

January 21, 2013, 09:37 AM
Hmmm..... Hidden gas block.... Sneaky sneaky
That is the way I understand it. Bushmaster invented the Dissipator and that is the way they did it so anyone else using mid length or rifle length gas system on a carbine length barrel and calling it a dissipator are selling some sort of immitation that isn't what the original intention of the dissipator was. Think about what the word dissipation means. It's about heat being dissipated and has nothing to do with gas system at all.

January 21, 2013, 10:26 AM
One of the reasons that the front sight is part of the gas block is that it makes the front sight assembly is stronger.

It needs that extra strength because the front sight is so high. That's related to the rear sight being so high. Which is why there is/was a carrying handle which is because the butt stock is straight which is because the bolt carrier group recoils straight back into the the buffer tube which is in the stock.

Pant, pant, pant.

I think this is supposed to end with, "And this is the house that Jack built" or something like that.

January 21, 2013, 08:37 PM
@Skans... Knew all of that and own ARs with FSB, railed gas blocks, and low pro gas blocks. I just wasn't aware that they hid a low pro under the hand guard on the dissipators and maintained a carbine length gas system.

You see an FSB and just kinda assume that, of course the gas tube goes in there.

January 21, 2013, 08:44 PM
How a dissipator is set up:


January 22, 2013, 06:13 AM
Ahh, thanks RidgeRunner! Now I see.

So, carbine length gas system, rifle length sight radius.

And that one is running an M-4 contour barrel nonetheless. I would have figured they would have gone with something else since machining the M-203 cutout is a waste on that particular setup.

January 22, 2013, 08:41 PM
I'm guessing they already had the M4 barrel and decided to make a dissy out of it.

January 22, 2013, 10:24 PM
So...the front sight is part of the gas system then?

This is usually on the AR's with the A2 front site post. This is not always the case as seen in the pic above, and also if you go with an upper without a A2 post and a longer handguard/quadrail.

An example the AR15 on the left has the standard A2 post with the shorter sight radius, while the one on the right has a 13-inch handguard (no A2 sight) which would allow a longer sight radius if you choose to install back-up Iron sights.


Both are 16-inch barrels but would have different sight radius lengths.

January 23, 2013, 12:09 AM
Good info. Thanks guys!

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