For my 16" middy with F-base to be BSZ sighted (zero at 50, +1.5" at 100) with either a folding Troy or fixed LT rear, the post must be flush with the top of the protective wings. So the bottom of the post is maybe 1/16" higher than it should be. This is obviously not ideal, but I’d like to know if it is acceptable. I am also curious as to why this would be the case. Windage is within spec, so I don’t think it’s canted. Sorry if I missed a FAQ or sticky which has the obvious solution!
I have a Troy folding front sight with wings and a Troy Diamondhead rear folding sight on a full rail LWRC. I thought it was a little crazy because my front post is just slightly higher than the wings for a 50 yd. zero. Hmmmm, maybe I’m not alone.
I know there are two different height front posts. One is to compensate for a non-F marked base.
What size groups are y’all shooting?
Colt currently offers four different front sight posts for use on their AR-15/M16 family of weapons. The front sight posts can be 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 nominal height of 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 height of this post is also 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 height of 0.260”.
SP61706
The last sight post is Colt’s tall sight post which has a nominal height of 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 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 1.25 MOA of evlevation.
Bushmaster makes a front sight post that has a nominal height of 0.310”.
The front sight post on a standard Colt front sight base will bottom-out in the sight well at approximately 24 clicks below flush. On an “F” marked front sight base the sight “shelf” is a whopping 0.040” higher than on a standard front sight base. That means you might have to turn your front sight post down approximately 6 clicks further with an “F” marked front sight base than on a standard front sight base. (6 whole clicks difference. Oh, the inhumanity!) So unless your front sight post is set at 18 clicks below flush with a standard front sight base, using an “F” marked front sight base will be a non-issue.
I have an F-marked FSB. I think it has the correct post for that base, since if the top of the post were reasonably lower than the top of the wings, the flat “base” of the post would be level with the horizontal area on the FSB, as it is with my other AR.
If I bought a taller post, I don’t think that would fix the problem, since the top of the post would still be too high, even if the bottom were correct.
I just happened to think, I never did flip to the large aperture, since I always sight with and use the small one (except when folding down, thanks to Troy design). I don’t recall how this affects POI, however even if it made the correction, I always use the small one, so I don’t think that helps either.
BTW, today I shot about 1.5" groups at 50 and a 3" group at 100 with irons. This is its 3rd time out and maybe 150 rounds total. Only one failure (to eject), and I don’t think it’s a coincidence that was one of the few times I wasn’t using a p-mag. I am happy with the accuracy, since it is a lightweight barrel, and I’m sure it is more accurate than I, especially with irons. However this front post issue is disturbing…
My reloads are 25 gr. H335 with RP case, CCI450 primers and SS109 bullets. They measure just under 1.5" at 50 yds. My 24.8 gr. H335 load measured around 1.25". This is with the iron sights mentioned earlier. I guess I need to throw in some PMC 55 gr. just to see where it is printing as compared to these loads.
Shot again today and threw the LT BUIS back on. Had to crank the windage over from use with the other AR, but the elevation was unfortunately the same as with the Troy. I use only factory loads, and today it was Fiocchi 55 gr and Black Hills Blue box, 68 or 77, I forgot which. Both 1.5" groups, both needed the front post right at the top of the wings to be zeroed at 50 yards.
Anyone know what the deal could be here?
I’ve read about bad “F” marked FSB, seems like it was a BM. Seems like that could be your problem.
Could you get a measurement from the top of the barrel to the top of the “shelf” on the FSB, like Molon shows in his pictures? Should be able to use the depth gauge out of a dial caliper.
Let us know what you find.
I’ve been trying to read up for you and it is “cornfuzing” the heck out of me. I read that both a “F” marked and a standard FSB have the same OA length. (Don’t really understand why both types of FSB’s would be the same OAL but …?)
If that is the case, then if the pin is at the top of the guard wings on one it will be at the top of the guard wings on the other…??
In other words…I don’t know but I’m anxiously waiting to see what the solution is. 
.
carbinero, I have a couple uppers that require the FSP to be raised to near the top of the protective wings, to obtain a proper zero. Tolerance stacking of the upper receiver, barrel extension to barrel, and FSB (whether “F” or not), can be to blame. While I would rather see the FSP be well below the top of the wings, it doesn’t really hurt anything. But, I do usually throw in a Bushmaster .040" taller FSP on these uppers, so the flange sits closer to where it should be, and a bit of the wobble is eliminated. I would put in a taller FSP, and not worry about it, if I were you. Top of the FSP is still going be at the top of the wings, but the base will be closer to flush with the shelf in the FSB.
Only “solution” would be to try swapping to another upper receiver, I have seen this help. It may or may not, depending on where the stacking is having the greatest effect. Otherwise, to get your zero, the top of the FSP is always going to at the same height in relation to the wings, no matter what height post you use. Again, I wouldn’t worry about it, unless the post is actually above the wings.
Hello,
Look at the photo above that shows both entire FSBs side by side and you’ll see that the difference is in the F’s shelf height, ie. the flat surface between the ears isn’t machined as far down as the standard one is.
Thank you all for looking into this. Obviously others have had this issue as well. I will try to measure it, however I don’t know if I have the tools to discern a 0.04 difference. Further, if the ears are indeed the same height above bore, this would not solve the problem. As above, I’d rather have it a little low than a little high, and if just move on, I may get a taller post in the future. For now I don’t think the wobble per se is significant (although noticeably different from my other upper).
Protective wings being what they are, what are the odds I’d drop the upper square down such that the post gets dinged? Dunno. With that logic, 1 or 2 clicks above flush with top of the ears shouldn’t matter. Any more than that, though, and I think it would mess with the eye’s natural POA.
I am looking for a way out of contacting the vendor and/or manufacturer and shipping it back. However I do use irons as primary, so I want it set up right. As with the “mil spec” definition wars over tolerances and perfection, that’s what I’m asking: “what is acceptable.” So I have one vote above for just living with it. Any other opinions or concerns? Thanks again…
Added poll… :help:
I wouldn’t worry about it for many reasons. First would be dropping the weapon upside down and it landing square on the front sight post.
If you do this, you just exhibited poor weapons handling and are most likely a danger to yourself and others around you. You should also purchase a lottery ticket (as you would most likely win). 
I am looking for a way out of contacting the vendor and/or manufacturer and shipping it back. However I do use irons as primary, so I want it set up right. As with the “mil spec” definition wars over tolerances and perfection, that’s what I’m asking: “what is acceptable.” So I have one vote above for just living with it. Any other opinions or concerns? Thanks again…
Shooting irons is good. Your setup sounds just fine and you will most likely add a RDS at a later date and all of this really won’t matter much.
C4
Thanks Grant.