I was sighting in my RRA a few weeks ago with my aimpoint and larue BUIS and I needed to almost pop the front sight out because it was too short to use with the correct mil height rear sight.

so I guess my question is:
Why do companies use incorrect height FSB when they KNOW you are going to be using some sort of buis on their flattops?

I guess they just don't feel like having to keep track of A2 FSBs and F marked FSBs.  And if the FSP isn't high enough, you get to come back and buy one.  

Good question. That ain't right. Maybe a shorter detachable rear sight.

Quoted: Good question. That ain't right. Maybe a shorter detachable rear sight.

Problem is most, if not all, BUISs are made for F marked front sight bases.  I got lucky on my Bushmaster and it just barely works with my ARMS #40A2.  

It's a PITA, that's for sure, but it's also easily rectified by ordering some  taller front sight posts from BM for all of $4.95/post.  Get a couple along with some spare parts to help defray the shipping charges.

They do, its only dolt that came up with the "M4" FSB. Just another lame attempt at being "different" .

They do it to cut costs and save $$$.  I guess it's much easier and cheaper to make a taller front sight post...

I think the more fundamental question is "Why did Colt decide to make second style of FSB with a different height?"

But given the facts on the ground, I think the answer is that it is cheaper to change out the front sight post than to stock and assemble two different styles of front sight bases.

For what it is worth, I have read that the forgings, and thus the overall ear heights, are same on both F and non-F front sight bases.  The only difference is how deep the flat is milled.  Assuming that this is correct, there is nothing wrong with simply substituting a taller front sight post.  It won't be any less protected than a post in an F front sight base would be.

Quoted: For what it is worth, I have read that the forgings, and thus the overall ear heights, are same on both F and non-F front sight bases.  The only difference is how deep the flat is milled.  Assuming that this is correct, there is nothing wrong with simply substituting a taller front sight post.  It won't be any less protected than a post in an F front sight base would be.

Did not know that. Might as well go with the taller post. I still think if a shorter rear buis would be a desirable option, when handguard prices range from $20 to $400 it would appear that parts selection is not always based on economics.

The original thread where I read about the difference being how deep the flat is milled seems to have been lost (probably archived), but you can see a cached copy in Google here

The relevant portion is this:

Quoted: Colt’s flattop uppers require a higher FSB; this much is provably true. This is an “F” marked FSB. The “F” is the Flattop mark. Any Colt barrel destined for use on a flattop upper will/should have this mark. These FSBs are the same height as standard FSBs the difference lays in how deep the “UP” stamped flat is milled. The protective ears are noticeably shorter on the “F” marked FSBs.

(Red highlighting added by me)

As you can see, the information came from a knowledgeable and reliable source, Tweak, not from an anonymous Joe like me.

Quoted: The original thread where I read about the difference being how deep the flat is milled seems to have been lost (probably archived), but you can see a cached copy in Google here The relevant portion is this: Quoted: Colt’s flattop uppers require a higher FSB; this much is provably true. This is an “F” marked FSB. photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=16149 The “F” is the Flattop mark. Any Colt barrel destined for use on a flattop upper will/should have this mark. These FSBs are the same height as standard FSBs the difference lays in how deep the “UP” stamped flat is milled. The protective ears are noticeably shorter on the “F” marked FSBs. photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=31840 (Red highlighting added by me) As you can see, the information came from a knowledgeable and reliable source, Tweak, not from an anonymous Joe like me.

Having been accused in the past of not knowing what I was talking about, I expect that discretion should keep me from posting this yet again---but I always have had a big mouth.

AR type carbines have ALWAYS had a higher front sight point. My 1978 issue SP1 carbine came with one. My buddies who were Ordnance armorers during and just after Viet Nam have confirmed that their shops stocked two different height front sight inserts; one for rifles and a higher one for the XM177s.

In other words, it's got nothing to do with flat tops, or add on optics. If it's a carbine or SMG type AR, it needs a higher front sight point. My unscoped Rock River barrelled M4forgery, with a Bushie sight assy, needs one; as does my XM177E2 clone w/an A1 type upper.  

Up until fairly recently, Colt did what many of us have had to do.  They used a higher sight insert. I suspect, and at least one currently serving GI has confirmed, that the F marked base was incepted simply to let Ordnance use a single height post on all M16s, and M4s.

Now everyone on this sight has a right to believe anything they want to, and to post any reason for anything, but the simple fact is that all those who want to attribute the F marked base to some special "flat topness"on the M4 have totally ignored my multiple postings about short barrelled pieces having always needed and been fitted with higher front sights (whether with higher posts or F bases).  Some folks just have a problem fiding out they made a simple mistake I guess

Okay.  I'll bite.  Does this (highly exaggerated and simplified) diagram explain why a carbine front sight must be higher than a rifle front sight no matter which type of upper receiver is used?



That would make sense, and I could believe what you say.

Regardless of why the carbine front sight needs to be higher, do you agree that in practice the forging itself is the same height and the only difference between F and non-F front sight bases is the depth of flat?  Or in your experience is that wrong also?

Quoted: In other words, it's got nothing to do with flat tops, or add on optics. If it's a carbine or SMG type AR, it needs a higher front sight point. My unscoped Rock River barrelled M4forgery, with a Bushie sight assy, needs one; as does my XM177E2 clone w/an A1 type upper.

WRONG.

You keep bringing this up and you are consistently proven wrong.  

Quoted: Up until fairly recently, Colt did what many of us have had to do.  They used a higher sight insert. I suspect, and at least one currently serving GI has confirmed, that the F marked base was incepted simply to let Ordnance use a single height post on all M16s, and M4s.

This is true, but it applies to flattops vs. fixed handles, not carbine and rifles.

Quoted: Okay.  I'll bite.  Does this (highly exaggerated and simplified) diagram explain why a carbine front sight must be higher than a rifle front sight no matter which type of upper receiver is used? img.photobucket.com/albums/v384/zrxc77/fsb_heights.png That would make sense, and I could believe what you say. Regardless of why the carbine front sight needs to be higher, do you agree that in practice the forging itself is the same height and the only difference between F and non-F front sight bases is the depth of flat?  Or in your experience is that wrong also?

Your diagram doesn't fly because a carbine and rifle will never have the same POA/POI because the bullet is moving at different velocities due to barrel length, thus a different flight path.

In your diagram, you have the POA is the same, but the POI will not be the same.  

If the POI is the same, the POA will not be the same.

OK now I'm really F'd up

Quoted: Your diagram doesn't fly because a carbine and rifle will never have the same POA/POI because the bullet is moving at different velocities due to barrel length, thus a different flight path. In your diagram, you have the POA is the same, but the POI will not be the same.

I understand that.  That is the reason I stated that the diagram is simplified.  I don't have a ballistics calculator to plot an exact trajectory.  I was simply trying to illustrate what I thought might be the first-order reason for needing different front sight heights as asserted by shamayim.  I was (and am) trying to understand the different arguments and thought that an illustration might be useful for people to say "Yes, that's what I meant" or "No, that's not what I meant."

Rather than dismissing the diagram out of hand, can you supply exact figures that would indicate it is bogus?  I don't have a good feel for the relative magnitude of the bullet trajectory versus the simple sight geometry.

$4.95 and your problems go away people........

Quoted: $4.95 and your problems go away people........

Agreed.  I just want to know why.

look for posts on the carry handel colt vs others there your answers are

For shamayim's theory to be correct, the nominal rear sight height would have to be higher than the nominal front sight height. This would make zrxc77's exagerated drawing correct, as well. This explanation also assumes that the over-bore axis nominal height of the A2 rear sight is the same as the A3 CH's sight.

If, OTOH, the rear sights are different heights - that is the only reason for the front sight post to be higher.

This is relatively easy to figure out. Who has the correct measurements?

Quoted: For shamayim's theory to be correct, the nominal rear sight height would have to be higher than the nominal front sight height.

The nominal rear sight height (as measured over the bore) is higher than the front.  That isn't in doubt.  You always have to tip the barrel up to give the bullet a positive trajectory to counteract the effects of gravity.*

Quoted: This would make zrxc77's exagerated drawing correct, as well. This explanation also assumes that the over-bore axis nominal height of the A2 rear sight is the same as the A3 CH's sight.

The drawing was specifically intended to illustrate a possible need for different front sight heights between rifles and carbines even when the rear sights are the same height.  Don't confuse the drawing with the fixed/detachable carry handle question.  For the sake of clarity and simplicity, assume that both the rifle and carbine pictured in the diagram are both fixed carry handle A2 style uppers.

Quoted: If, OTOH, the rear sights are different heights - that is the only reason for the front sight post to be higher.

I disagree.  Sight geometry (as illustrated) and trajectory differences caused by the change in velocity with barrel length (as scottryan emphasized) are also possible reasons for different front sight heights.

Regarding shamayim's contention and those who dispute it, I think a couple questions need to be answered before one could judge the merits of the conflicting views:

1.  How much does the need to raise the carbine's front sight due to simple geometry considerations compare to the need to lower it due to velocity induced trajectory changes?  I believe I could figure out a reasonable estimate of the change required by the geometry, but I don't have a ready means to calculate the change required by the trajectory differences.

2.  Is the net change in height from question one large enough to warrant a change to the FSB, or would it fall within the normal adjustment range of the front sight post?

Quoted: This is relatively easy to figure out. Who has the correct measurements?

You would think so, wouldn't you?  But in the year and nine months I have been lurking here I have yet to see an unequivocal, absolutely indisputable resolution of this question.  I am relatively new to firearms, but I have already seen that there seems to be a distressingly large amount of this type of confusion in the firearms field in general.  There is too much speculation and too many people (myself included) with too many opinions, but not enough unassailable fact.  I don't know why this is, but it is what I have observed.


* Assuming the nominal case where your target is at the same level as you.  In other words, you aren't shooting downhill at a target below you.

If I'm understanding that Colt A3 uppers are differnet than other "A3" uppers then you're telling me that there might be a problem with my ARMS 40 BUIS and my PRI Flip up front site on my Colt 6721?


Then also is my Colt "carry handle" that i took off the 6721 not going to work properly with my FN A3 upper I bought that didn't come with a "carry handle"

I really think we need to take more measurements and figure this out.  I'm just trying to figure out how to "measure" it.

Quoted: $4.95 and your problems go away people........

And in some cases free.  Call Bushmaster and tell them, I have heard of people having them sent for free to fix the problem.  Worth a try anyway.

Colt/milspec detachable carry handles have the rear sight sitting .040" higher than aftermarket.

F marked bases are spec for flat tops.Regualr FSBs are also spec just for fixed handle A1 or A2 rifles and carbines.