I just finished my 733 build and it’s perfect. I took it to the range today and it was shooting low. As I started to turn the front sight post clockwise following the arrow that says up. I noticed an improvement as I continued to turn the front sight post to get it sighted in I bottomed out the sight post threads. I ordered a standard FSB and had it installed on a blank barrel. I’ve double checked my order and that is what I bought. Could I have gotten an unmarked F height FSB. Is my thinking correct. An F marked base would place the front sight post to high. Thus the need to lower it. What do you think. P.S. I did get it on the bullseye.

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It doesn't matter if you did with regard to being able to obtain a correct zero. The FSB doesn't affect the height of the top of the front sight post above the bore, within normal limits.  The main difference between a "standard" front sight base and an F marked front sight base is the height of the "shelf" of the front sight base above the barrel.  The F marked front sight base has the higher shelf so that the "flange" of the sight post isn't sitting above the shelf when raising the sight post (bringing the point of impact lower) while zeroing.

(The dimensions in the pics below are approximate measurements taken with the depth gauge portion of digital calipers.  The exact specifications can be seen in the drawings of the TDPs.)









Do you have a 0.625" or 0.750" front sight base?  Did you chase the threads in the well of the front sight base? How low is the top of the front sight post sitting in the front sight base?  What is the measurement of the the "shelf" of your front sight base above the top of the barrel? What is the measurement of the top of the front sight post to the center of the bore?

At what distance were you attempting to zero?  What sight picture were you using?  What front sight post did you put in the front sight base?


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 of the post above the "flange", (short or tall.)  The height of the posts were measured from the top of the sight post down to the top of the flange of the sight post using the depth gauge portion of digital calipers.

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



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



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



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



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



Here is a pic with the round sight post and the tall sight post side by side for comparison.




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 evlevation (with the rifle length sight radius).

Thanks MOLON. That’s a lot of information. I’ll have to start doing some measurements. I’m using .625” barrel. And I’m getting on paper at in indoor 25 yard range. Kinda short I know. All I did to get to where I am is buy a standard FSB and a standard sight post. I was careful not to mix it up with the .040” taller post I have. I’ll start looking more in-depth into this situation.

You said that the post was "bottomed out."  Is the post really sitting this low?




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Yes it really is.
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Chop your post in half with a Dremel tool, this will solve your problem.

Agreed. I’m just checking in with the community first.

Can you take a picture with the front sight base aligned level from the shooters view?

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That's not going to solve the actual problem if his front sight post really is this low in the well . . .




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I can surely take that picture but I not really sure what you mean. can you give more details on how you want the picture?

Like this one from the shooters perspective



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Compared to Molon's pic it looks like you've still got a ways to go before being completely bottomed out. But, you still probably have an unmarked "F" height FSB on there. I have three Brownells 20" 1/7 A1 Retro barrels from a few years ago that required me turn the front sight post down into the FSB similar to yours if not a bit lower. I suspect the barrels to have unmarked "F" height FSBs as well.

What range did you zero at? I sighted in at 50 yards with the barrels I mentioned.

So your front sight post is obviously not "bottomed out."  If the post won't go any lower, there's multiple problems that could be causing that.

First remove the front sight post and check the post for manufacturing defects.  Even if you don't see anything obvious, I would try a different sight post.  Then check the threads in the sight base for burrs.  You can try chasing the threads in the base with a #8-36 tap; Brownells often stocks these.

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I’ll try all of that. Thanks. I’ll have to do that then make it back to the range.

excellent post!

I took the front sight assembly apart. Mine had the tall detent. I swapped it out for the short detent. My sight post goes all the way down now. Using this detent I can most certainly sight in the rifle. I guess it will just have a low looking post

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That still seems really weird.

Is your barrel canted slightly?
Have you run a truing tool across the front of your upper receiver?
What is the rear sight you are using?
How is it set currently?

Bizarre... I'm not sure I've ever seen a front sight detent so short next to a front sight detent so long. I've seen slightly longer than normal ones, but usually they are between the length of the two in your picture.

How low were you actually hitting at 25 yds compared to your point of aim?  Didn’t see that mentioned, kind of important…

Because for what I prefer (50 yd zero), your 25 yd POI should be about an inch low anyhow.  Often a decent method of getting close if you don’t have access to a longer range quite yet is to just zero 1” low, then dial it tighter in at 50 when you get a chance.

You hopefully already know this, but plenty of people might try for POA/POI at 25 yds, and that would not be the way to do it, and would exasperate the situation.  

Played the standard front sight base on 16” rifles game many times with batches of LE Bushmasters that they shipped out 15-20 years ago.  When you have a lot of them it it can be problematic that they did not use F bases on a few of them, most were fine….  And we issued Trijicon front and rear sights, so you didn’t have an option for a taller front sight either.  Some were barely retained they were so high in the base…not cool.  I always figured BM just had a pile of regular FSB’s and was trying to use them up.

Anyhow- how low were you?

I think that the Trijicon front sights might have come with shorter detents like that, as the sight was actually a rotating sight disk, which spun on top of another rotating disk.  Had to be that way so the tritium dot could always face the shooter.  But it caused the sight to be kind of deeper in the base/round section, so I think that is why the shorter detents.  Could be recalling incorrectly, been forever since I installed one of those things.  Haven’t bothered with night sights on the irons since I started using a red dot.

Makes sense, otherwise it's pretty odd ball. Not sure where the OP got a front sight post detent so short?

Hey tac556 I was hitting paper about 3-4" below that bullseye. but I agree that 25 yards is short I like your idea of sight in 1" low at 25 yds. I really appreciate your help. I  don't shoot much as I'd like but I'm trying

I went back to the range today. After a good inspection and sight post change. I’m shooting 11.5” 5.56mm barrel with a CMMG .22lr drop in conversion. It sighted in great. Not low at 25  yards. Sight post looks about the same if not a small amountt higher in the sight base. I put 300 bullets down range. All pretty good and on target.

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