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Posted: 4/14/2015 2:26:37 PM EDT
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Ordered 20 inch flat top upper from one of the vendor members. Had to screw post way up to zero, sent it back to them for evaluation and they put longer post in FSB.
I asked them: why doesnt this upper have F marked fsb? Reply was that they have found them to be unnecessary. My BCM 20 inch flat top came with F marked fsb. So- anybody got any experience with this issue? Advice? Thanks. |
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This is how it works.
A flat top uses an F marked FSB. An A2 upper uses a non F marked FSB. An F marked FSB is .04 taller than a non F marked FSB. So if you have a flat top with a non F marked FSB, You need a .04 taller front sight post to meet the proper mechanical zero height you will need. |
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longer front sight post is not sticking up past ears, but not in the ball park with the way my other 5 ARs (with standard front sight posts) look after zero.
and this is with my FN carry handle. Which brings me back to question of those that have experienced this: If the correction is-F marked fsb or longer front sight post, which option is acceptable to Yall? |
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Quoted:
longer front sight post is not sticking up past ears, but not in the ball park with the way my other 5 ARs (with standard front sight posts) look after zero. and this is with my FN carry handle. Which brings me back to question of those that have experienced this: If the correction is-F marked fsb or longer front sight post, which option is acceptable to Yall? Either one. Of course swapping for an F marked would be optimal, but you would have to redrill the new F marked FSB with the barrel that already has taper pin holes (those wont work for your new FSB unless you get super lucky tho) I have zero issues with my taller front sight post either. |
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The F-marked FSB was developed to allow the use of the same front sight post regardless of whether the gun has a carry handle or a flat top. They apparently use the same raw forging and just mill the sight deck 0.040" higher...
In the military realm, there's no issue: only 14.5" barrels need the F-marked FSB, while 20" barrels get the "unmarked" FSB. It's us PITA civilians who want 16" barrels in flat top receivers that cause the problems. 
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Quoted:
longer front sight post is not sticking up past ears, but not in the ball park with the way my other 5 ARs (with standard front sight posts) look after zero. and this is with my FN carry handle. Which brings me back to question of those that have experienced this: If the correction is-F marked fsb or longer front sight post, which option is acceptable to Yall? All I care about is that the front sight does not stick up proud of the protective wings. The stamp on the base is of no concern itself. |
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Quoted: The F-marked FSB was developed to allow the use of the same front sight post regardless of whether the gun has a carry handle or a flat top. They apparently use the same raw forging and just mill the sight deck 0.040" higher... In the military realm, there's no issue: only 14.5" barrels need the F-marked FSB, while 20" barrels get the "unmarked" FSB. It's us PITA civilians who want 16" barrels in flat top receivers that cause the problems. |
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Quoted:
M16A4's have F marked FSBs. Quoted:
Quoted:
The F-marked FSB was developed to allow the use of the same front sight post regardless of whether the gun has a carry handle or a flat top. They apparently use the same raw forging and just mill the sight deck 0.040" higher... In the military realm, there's no issue: only 14.5" barrels need the F-marked FSB, while 20" barrels get the "unmarked" FSB. It's us PITA civilians who want 16" barrels in flat top receivers that cause the problems. Correct. It's not about the barrel length, it's about the sight height. Flat top (regardless of barrel length) - F marked. A1/A2 upper (regardless of barrel length)- non F marked. |
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Quoted:
M16A4's have F marked FSBs. Quoted:
Quoted:
The F-marked FSB was developed to allow the use of the same front sight post regardless of whether the gun has a carry handle or a flat top. They apparently use the same raw forging and just mill the sight deck 0.040" higher... In the military realm, there's no issue: only 14.5" barrels need the F-marked FSB, while 20" barrels get the "unmarked" FSB. It's us PITA civilians who want 16" barrels in flat top receivers that cause the problems. A4s have flat top uppers AND 20" barrels. I wasn't thinking about the A4 when I OVER generalized above. My bad. |
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Quoted: A4s have flat top uppers AND 20" barrels. I wasn't thinking about the A4 when I OVER generalized above. My bad. Quoted: Quoted: Quoted: The F-marked FSB was developed to allow the use of the same front sight post regardless of whether the gun has a carry handle or a flat top. They apparently use the same raw forging and just mill the sight deck 0.040" higher... In the military realm, there's no issue: only 14.5" barrels need the F-marked FSB, while 20" barrels get the "unmarked" FSB. It's us PITA civilians who want 16" barrels in flat top receivers that cause the problems. A4s have flat top uppers AND 20" barrels. I wasn't thinking about the A4 when I OVER generalized above. My bad. All good. It's argued whether they NEED them. But they do in fact have them. |
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