F-Marked FSB

[Blob]

From my understanding of the Chart's explanation:

Front sight bases come in two basic varieties. One is the front sight base intended for use on carbines and rifles with fixed A2 uppers,the other (The "F") is intended for carbines with flattop uppers. In order to ensure compatibility with the various aftermarket rear back up iron sights

f-marked FSBs are meant to be used with carbines that have flattop uppers. Would a shorter, non f-marked FSB be more desirable with a 20 inch rifle with a flattop?

Also, how much would a non f-marked FSB affect the zero on a 14.5" carbine?

I'm not planning a build or anything, just want to learn.

[AMMOTECH]

Non-"F" marked is for solid carry handle with A2 rear sight. Works with both 16" or 20". If the 20" is a flat top then you need the "F".

I have a non- "F" on a 14.5 Bushmaster; It zeroed fine @ 25m with that set-up but when I tried to zero at 50Y I had to turn the sight post up so high that it looked and felt like it might come out. I replaced the post with a longer one and have not had any problems with a 50y zero using a BM cut carry handle as a BUIS.

Flat tops need an "F"
Non- flat tops do not.

[citizensoldier16]

I too had a non-F marked FSB on my BM 16". I called BM and complained that they were manufacturing rifles that would not zero with BUIS installed instead of their carry handle. They promptly sent me a taller front sight post for free. I installed it and it works just fine. It is noticably taller, but does not stick out past the front sight arms.

Just saying that the taller post is an option if you do not wish to replace the non-F marked FSB. If you're building a rifle, do it right though.

[Frens]

I was under the impression that the F-FSB was for cabines (both with fixed CH and flat top)

[Blob]

What I really want to know is if F-marked FSBs are a necessary feature on a 20 inch rifle with a BUIS (not carry handle), or if the extra velocity from a 20 inch negates the necessity.

[RAM Engineer]

What Ammotech said.

Flattop Receiver (regardless of detachable carry handle or BUIS) = you need "F"
Fixed A2/carry handle = non-"F"

[fdxpilot]

What I really want to know is if F-marked FSBs are a necessary feature on a 20 inch rifle with a BUIS (not carry handle), or if the extra velocity from a 20 inch negates the necessity.

I don't think velocity has much to do with it. It's all about angles. If you have the short FSB, the initial trajectory of the bullet will be well above horizontal. Not a big problem if you are trying to zero for a short range, like the 15yd mentioned above. Past that 25yd, though, the bullet will be well above the point of aim line . Higher velocity just means the bullet will get higher above POA and stay above it longer.

However, if you are trying to zero at 50, 100, or 200 yds, the front post may not raise enought to allow bullet to hit POA.

[Blob]

I don't think velocity has much to do with it. It's all about angles. If you have the short FSB, the initial trajectory of the bullet will be well above horizontal. Not a big problem if you are trying to zero for a short range, like the 15yd mentioned above. Past that 25yd, though, the bullet will be well above the point of aim line . Higher velocity just means the bullet will get higher above POA and stay above it longer.

However, if you are trying to zero at 50, 100, or 200 yds, the front post may not raise enought to allow bullet to hit POA.

The extra velocity means it will drop less at longer ranges, and I'm wondering if that's enough so that you don't need the extra 0.04 inches of sight post height.

[fdxpilot]

The extra velocity means it will drop less at longer ranges, and I'm wondering if that's enough so that you don't need the extra 0.04 inches of sight post height.

That just makes the problem worse. The short sight already has the bullet high, and the higher velocity just keeps it above POA higher and longer. Raising the sight post gives less angle between the barrel and POA, moving the zeroed distance away from the shooter. Once you get to the max height on the sight post, you are at your max Zero distance. Shooting anything further away will mean using holdover until the bullet actually descends through the POA line again. Admittedly, this is true for any zeroed distance, but a non-F FSB in a flat top carbine will have a fairly close maximum zero. It will resemble the red line in this chart.



The chart is from this thread, which has a lot of good info on zeroing an AR.

https://www.m4carbine.net/showthread.php?t=34671&page=5

[rob_s]

"F" is for "flat top", regardless of 10.5", 16", or 20".

Sorry if the E of F is confusing in this regard.