Standard FSB with flat top receiver

[Spiffums]

Can you just get a F front side post and put it in a standard FSB? I'm screwing around with a Bushmaster that came from factory with Fixed carry handle. I got a flat top upper receiver and put the barrel in it. It has a factory perm attached phantom flash hider so it's not just an easy "change the FSB".

[opngrnd]

I could have sworn Molon previously shared a graphic on this. I'd bet you can do with a taller front sight post and make things work, but first I'd go to the range and see how far off you are.

[jackblack73]

KNS Precision sells a taller front sight post for use with a non-F marked FSB.

[Molon]

I could have sworn Molon previously shared a graphic on this.

Colt Front Sight Posts

Colt has produced at least four different front sight posts for use on their AR-15/M16/M4 series of weapons. The front sight posts can be roughly categorized according to their basic shape, (square or round) their profile, (straight or tapered) and their height, (short or tall.) The height of the posts are measured from the top of the sight post down to the top of the “flange” of the sight post.

The sight post with the part number SP64507 has a short, square and straight configuration. This post has a measured nominal height of approximately 0.270”

SP64507
https://www.ar15.com/media/mediaFiles/28568/short_straight_front_sight_post_01_resiz-3192734.jpg


Part number SP64665 also has a short and square post, but differs from the above sight post in that it has a tapered shape to it; broader at the base and narrowing at the top. The measured nominal height of this post is also approximately 0.270”.

SP64665
https://www.ar15.com/media/mediaFiles/28568/short_tapered_sight_post_01_resized-1302-3192739.jpg


Shown below are the square, short/straight and short/tapered sight posts side by side for comparison.

https://www.ar15.com/media/mediaFiles/28568/short_straight_and_tapered_sight_posts_0-3192757.jpg

The sight post associated with the M16/M16A1 series of rifles has a round shape and is also short and tapered. The part number for this post is SP61706. It is slightly shorter than the two posts described above with a measured nominal height of approximately 0.260”.

SP61706
https://www.ar15.com/media/mediaFiles/28568/round_front_sight_post_01_resized-130204-3192766.jpg


The last sight post is Colt’s tall sight post which has a measured nominal height of approximately 0.300”. It has a square and straight configuration. Its part number is SP62447.

SP62447
https://i.ibb.co/74dHCDB/colt-tall-front-sight-post-01-resized-1302038.jpg



The original round sight post has five equally spaced notches around the sight post flange for elevation adjustment. Each notch corresponds to approximately 1 MOA of elevation (when used with a rifle length sight radius). The other three sight posts all have four elevation notches on their flanges corresponding to approximately 1.25 MOA of elevation (with the rifle length sight radius).

https://www.ar15.com/media/mediaFiles/28568/elevation_notches_on_front_sight_posts_0-3192773.jpg



Bushmaster makes a front sight post that has a nominal height of 0.310”.

https://i.ibb.co/JQs2d0L/front-sight-posts2-1-1302043.jpg


…

[Uncas47]

Thank You.

[opngrnd]

Thanks, Molon!