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View Full Version : A2 vs. 'F marked' front sight


plumbum
05-29-2011, 8:39 AM
My stupid question of the day:

What does it mean when a company says it will supply an upper with an 'F marked' front sight base? Is that the same height as an A3/M4 upper rail?

For instance, if I order an 'A2' style front sight, and then add a MagPul rear folding sight, they will line up the same as if I was using both a front (F marked base) and rear MagPul folding sights?

All these terms make my head hurt, I should go back to shotguns where I belong! :confused:

dieselpower
05-29-2011, 8:40 AM
F= Flat top (never confirmed but thats what its for) The height is a bit less on a A2 carry handle. .040 IIRC...low on coffee.



LOOK HERE (http://www.ar15.com/archive/topic.html?b=3&f=4&t=411826) <--- this has the info


easy search

missiontrails
05-29-2011, 8:50 AM
The triangle sight housings are the same size, but the post platform is around 1/4" higher on the F marked... my guess is to accommodate for the thickness of the picatinny rail on the flat receiver....

plumbum
05-29-2011, 8:55 AM
As always, great info, thanks!

dieselpower
05-29-2011, 8:57 AM
The triangle sight housings are the same size, but the post platform is around 1/4" higher on the F marked... my guess is to accommodate for the thickness of the picatinny rail on the flat receiver....

Thats one of those things we just assume and I have never seen a person confirm. We would need to measure from the center of a peep sight to the center of the bore of both to see.

I imagine different BUIS manufactures have tolerance stacking troubles we do not even know of. Would love a comparison.

Its all moot in the end since all FSBs have adjustable post. I have heard people say you can not zero a flat top with a standard FSB..it must be an F marked one. Thats freaking stupid. I have a flat top and a standard FSB, I zero just fine.

voiceoftheright
05-29-2011, 9:48 AM
Thats one of those things we just assume and I have never seen a person confirm. We would need to measure from the center of a peep sight to the center of the bore of both to see.

I imagine different BUIS manufactures have tolerance stacking troubles we do not even know of. Would love a comparison.

Its all moot in the end since all FSBs have adjustable post. I have heard people say you can not zero a flat top with a standard FSB..it must be an F marked one. Thats freaking stupid. I have a flat top and a standard FSB, I zero just fine.

Ya, I have never experienced or seen anyone have a problem zeroing regardless of FSB. I wouldn't worry about it, seems more like another marketing tool to add to the description more than anything.

missiontrails
05-29-2011, 12:57 PM
Ya, I have never experienced or seen anyone have a problem zeroing regardless of FSB. I wouldn't worry about it, seems more like another marketing tool to add to the description more than anything.

Umm, you don't want to mix an A2 FSB with a flat top receiver... have fun zeroing that.... you would have to unscrew the post All the way, and then some.....

dieselpower
05-29-2011, 1:07 PM
Umm, you don't want to mix an A2 FSB with a flat top receiver... have fun zeroing that.... you would have to unscrew the post All the way, and then some.....

The deck height is .040 higher on a F FSB. The center post needs to be 5mm and you are nearly dead nuts, maybe 1 or 2 notches either way for a zero. .040 = 1mm.

So with a A2 Rear BUIS on a flat top with a standard non-F marked FSB, you need to be 6mm. Bushmaster sells a center post that is 1mm longer (.040) longer so your center post starts at 6mm and not 5mm like an A2.

Anyway you do the math, it works out. It is a bit confusing at first, but you can zero just fine.