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Old December 21, 2012, 07:43 PM   #1
Ac1d0v3r1d3
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Ar-15 front sight

I've always wondered, why is the front sight on most ar-15's so far from the end of the barrel?
I could speculate, but it would only be speculation.
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Old December 21, 2012, 11:30 PM   #2
ToddG
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Because the front sight is placed over a hole in the barrel, routing gasses back to the bold carrier group to cycle the magazine. "Timing" an AR is figuring out how long the bullet takes from exiting the cartridge, to passing the gas hole, to exiting the barrel. Until the bullet leaves the barrel the gas passes through the hole, the sight base, gas tube and into the upper receiver to push the bolt back and cycle the gun is the only way for gas to escape (read: pressure, cause the bullet is still effectively plugging the barrel). The time from the bullet passing the gas hole to exiting the barrel (venting the gasses that were going back to cycle the bolt) is called the dwell time. A big design deal to get that timing just right.

Too close the the business end and there isn't enough gas routed to cycle the bolt. Too close to the back and there is way too much gas going in, over pressuring the system and gunking up the receiver. That's why mid-length AR carbines can be a challenge to get right.


An alternative on some rifles is to use a low-profile gas block and put a separate front sight further down the barrel. Longer sight radius without affecting the timing.
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Old December 22, 2012, 12:06 AM   #3
Ac1d0v3r1d3
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I understand that the gas black needs to be in a certain place, but why is the front sight always on the gas black?
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Old December 22, 2012, 12:31 AM   #4
Quentin2
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Quote:
I understand that the gas black needs to be in a certain place, but why is the front sight always on the gas black?
The front sight isn't always on the gas block but often is. The gas block has to be securely mounted to the barrel with no shifting or cant which would allow a gas leak or stoppage. The front sight also has to be securely mounted to the barrel with no shifting or cant which would knock off its mechanical zero. So it made sense to incorporate both systems into the front sight base.
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Old December 22, 2012, 12:34 AM   #5
Ac1d0v3r1d3
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So the answer is that it is convenient. Thanks guys.
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Old December 22, 2012, 08:54 AM   #6
CTS
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I think the reason is more likely that is the way it was originally designed. You see many these days with no front sight at all or a sight mounted on a picatinny rail on the free float hand guard but the ones that are sold to look like the original AR15 comes in the origninal configuration.
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