|
Forum Rules | Firearms Safety | Firearms Photos | Links | Library | Lost Password | Email Changes |
Register | FAQ | Calendar | Today's Posts | Search |
|
Thread Tools | Search this Thread |
January 20, 2013, 08:56 PM | #1 |
Senior Member
Join Date: October 10, 2006
Location: S. CA
Posts: 421
|
Another AR nooby question
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 | #2 |
Senior Member
Join Date: March 31, 2010
Location: Communist State of IL.
Posts: 1,562
|
I may be wrong here? I was!
__________________
NRA Life Member, SAF Member Last edited by Xfire68; January 21, 2013 at 10:23 AM. |
January 20, 2013, 09:34 PM | #3 |
Senior Member
Join Date: December 29, 2010
Location: Shoshoni Wyoming
Posts: 2,713
|
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 | #4 |
Senior Member
Join Date: July 30, 2011
Location: Savannah TN
Posts: 1,220
|
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 | #5 |
Senior Member
Join Date: March 31, 2010
Location: Communist State of IL.
Posts: 1,562
|
See I knew I should have kept my mouth shut! LOL I was not even close on that one!
__________________
NRA Life Member, SAF Member |
January 20, 2013, 09:47 PM | #6 |
Senior Member
Join Date: January 28, 2006
Posts: 1,482
|
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 | #7 |
Senior Member
Join Date: November 6, 2011
Location: NE Georgia
Posts: 1,070
|
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.
http://www.google.com/imgres?imgurl=...9QEwAw&dur=448 |
January 21, 2013, 12:10 AM | #8 |
Senior Member
Join Date: October 10, 2006
Location: S. CA
Posts: 421
|
So...the front sight is part of the gas system then?
|
January 21, 2013, 12:24 AM | #9 | |
Senior Member
Join Date: July 30, 2011
Location: Savannah TN
Posts: 1,220
|
Quote:
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. |
|
January 21, 2013, 03:53 AM | #10 | |
Senior Member
Join Date: April 10, 2012
Location: San Diego CA
Posts: 6,876
|
Quote:
__________________
If Jesus had a gun , he'd probably still be alive ! I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again . |
|
January 21, 2013, 08:27 AM | #11 |
Senior Member
Join Date: January 28, 2006
Posts: 1,482
|
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 | #12 |
Senior Member
Join Date: November 20, 2008
Posts: 11,132
|
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 | #13 | |
Senior Member
Join Date: November 6, 2011
Location: NE Georgia
Posts: 1,070
|
Quote:
|
|
January 21, 2013, 10:26 AM | #14 |
Senior Member
Join Date: June 19, 2012
Location: East Texas
Posts: 407
|
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 | #15 |
Senior Member
Join Date: January 28, 2006
Posts: 1,482
|
@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 | #16 |
Senior Member
Join Date: February 8, 2008
Location: Denver, CO
Posts: 1,925
|
How a dissipator is set up:
|
January 22, 2013, 06:13 AM | #17 |
Senior Member
Join Date: January 28, 2006
Posts: 1,482
|
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 | #18 |
Senior Member
Join Date: February 8, 2008
Location: Denver, CO
Posts: 1,925
|
I'm guessing they already had the M4 barrel and decided to make a dissy out of it.
|
January 22, 2013, 10:24 PM | #19 | |
Senior Member
Join Date: December 21, 2010
Location: Central FL
Posts: 1,360
|
Quote:
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 | #20 |
Senior Member
Join Date: January 7, 2013
Posts: 106
|
Good info. Thanks guys!
Sent from my iPhone using Tapatalk |
|
|