i have a DPMS flat top 16" upper with a standard front sight.
i used a GG&G BUIS on the flat top.

i could not get a zero, even after raising my front sight post way past flush.
so i figured i needed a taller front post.
i have seen that before with flat top rifles.
so i bought the Bushmaster (.040 taller)
i still can not get a zero???
it seems the post is still not tall enough?
i have seen/sighted alot of AR's - but never seen this?

Have you tried another fold down rear sight like a #40 or KAC 300meter, and then you can determine if it is the rear or the front sight housing. Another thing that might be a prob. is that the receiver might not be to spec. I'd go and find a friend with a Colt and start measuring what you actually got for a weapon thats mil spec. and what isn't compared to a Colt that have to meet mil spec......
Good shootin, Jack

Bushmaster part# 9349056-M [url]http://www.bushmaster.com/shopping/scopes/9349056-m.asp[/url] .040 higher.  I just ordered one a couple of days ago.

He tried that one dude, read the post.

I don't know the exact dimensions but I used a Trijicon front post when I put the ARMS #40 on my Bushy. It was taller than the stock Bushy post.

Good luck finding what you need.

Yeah, I did read it but posted my reply several days later and didn't re-read it.  [;)]

If you can't get it to zero with that sight post, you're likely looking at a problem with the rear sight not being low enough.  As 3rdtk recommended, try a different brand BUIS.

A little mission for this weekend.
I have a carry handle, GGG MAD, and ARMS #40.

Maybe one of those would work.

thanks!

I said the front sight housing was too low, read my post. The #40 is std mil spec ht. It's most likely the front sight housing is too low and obviously too low and I gave the full reason. I also asked who's flat top it was and if that is to spec as that is also a possibility. When you match non mil spec will mil spec, all kinds of things show up. Compare the weapon you have to a Colt gov't issue weapon and measure, you will no dobt see where the problem lies, my bet id the front sight housing that the front sight post sits in.
Good shootin, Jack

[url=http://ar15.com/forums/announcement.html?b=3&f=66&id=185]FAQ II[/url]

Look under "Excessive elevation"

Tweak is right on the money. I also read what some Colt distributor said and it is indeed nonsense. He needs to chech with the Co. before speaking beyond prices. The real solution is generally to get a real gov't mil spec front sight housing (base), but remember in doing that you want to make sure that the barrel is to the mil spec dia, not close, but to it! Othewise you can end up with gas exiting where you don't want it if the barrel is under size, or if the barrel is over size the front sight won't want to go on, the gas port hole need to also properly line up with the sight housing.
A little known history might give more insight (PUN) as to how things were when the first flat top M4's came out for delivory to the military. The first M4's were being evaluated at Ft. Bragg, and they noticed that they front sight post had to be raised so high that they were almost ready to fall out. The report to Colt developed into a first suggestion by COLT, that they make the detachable carry handle .040 higher to compensate for the elevation problem. That would have been a logistical nightmare, how in the world would the average soldier armorer keep them seperated in the shop and field, can't do something that dumb. If Colt had done the trig. in the first place, a prob. wouldn't have existed. The end result was the front sight housing was made .040 higher.  Since that was affixed to the barrel it would be easier to all concerned to keep sorted out, since it is not a QD item that can get mixed up in the field since it's pined to the barrel. An extended front sight post is prone to damage and certainly not going to be accepted by the military. It seems that Bushmaster use mostly ony the front sight housing for rifle length barrels, and that is the usually the culprit, but not always. Certainly the twist rate is not going to vary elevation to where the rd. goes like what the elevation of the sight is. I have also seen variations in receiver hts. that's why I mentioned previously mentioned it, but the receiver was not I.D. Another thing that needs to be mentioned is the fact that once in awhile you will find the bore not concentric to the out side dim's of the barrel. I have seen barrels at the chamber end start off centered, and then digress to being up to .040+ lower or higher, and left or right from center. These can also contribute to real difficult zero problems. Gentlemen, with zero problems, check the muzzle end of the weapon while you are checking the other things.
Hope this helps shed some light on what you get is quite often from what you pay.
Jack  

One question - is your DPMS a standard flattop is it one of their 'Lo Pro' flattops that have the rail higher than standard?  If the rail is higher than standard I could see a real problem trying to find a front sight that works.

the DPMS upper is a standard flat top.
carbine, H-Bar, fluted, crowned barrel, and
what looks like a standard front sight post.

thanks.

Correction: Sorry I mean't to say Colt wanted the make M4 the carry handle .040 lower, (not higher)! The rest is correct.
Good shootin, Jack

What [b]range[/b] are you trying to zero?

If you're trying 25 yards try 50, it's a better initial zero anyway (IBSZ).

-- Chuck

Chuck is correct, the Ft, Bragg eval. was at 100 meters. If it's a DPMS job shop lot, I would strongly suggest you get the receiver QC'd against a COLT which is military issue and genuine mil spec.
Jack

I was trying to get a IBSZ at approx 25m, then I normally goto 50, then check it at 100.

I am glad I started at 25, because at 50 - It was/is sooo far off - it wouldn't have hit the paper.

Also, What is "DPMS-job lot"?  
The DPMS Upper I have has DPMS 1/9 stamped on the bottom of the barrel (between the flutes).
The front sight base is one of the cleanest I have seen.  (kinda strange really) No cast or forging markings.  The finishing is on par to a upper or lower receiver. I have never seen that type of FSB before.  I an not sure if that is standard for DPMS.  This is their first Upper I have owned.

Am I incorrect, or isn't Colts' FSB taller than all other manufactures??
and are you saying, only Colts will zero w/ BUIS? (what other manufacture would be genuine "mil-spec")?
Thanks for your help.

Colt M4 front sight housings are at the correct ht. the others that are not at that ht. are too low and do not meet the gov't spec for a military issue weapon. That's why sometimes they want to sell you a taller front sight post, because their housing is to low and doesn't meet spec.
DPMS is not a mil spec maker, they job their stuff out to low bidders. Colt goes thru gov't inspection, DPMS does not.
Jack

thanks - 3rdtk.

Do you know (off hand) if there are other manufactures that build (specifically flat tops) to "Mil-Spec" (ie-taller FSB)

My FSB must really be extra low, because I would have guessed the taller (bushmaster +.040) front sight post would have allowed it to zero?

[img]http://photos.ar15.com/WS_Content/ImageGallery/Attachments/DownloadAttach.asp?sAccountUnq=19876&iGalleryUnq=1187&iImageUnq=20957[/img]

[img]http://photos.ar15.com/WS_Content/ImageGallery/Attachments/DownloadAttach.asp?sAccountUnq=19876&iGalleryUnq=1187&iImageUnq=20958[/img]

3rdtk,

My first carbine, (after the Corps)... I should say civilian carbine was/is a Colt (A2 Gov.)-1987.
I was kinda disappointed because it was not similar to my "issued" rifles.  Meaning -- no mag fence, improper front piviot/takedown pins, improper front receiver hold size.  Little stuff like that.

I have seen some Colts with those issues fixed.  Maybe those are the newer ones?  

Here is my question:  Are the new Colts sold to civilians, built to the same spec as the ones issued to the military? (not including all the obvious stuff like, AWB features, select fire, receiver blocks, etc.)
After looking at their website, some of the general specs for the M16 familiy are different for Military & Civilian.  For example, the site does not mention chrome lined bore, chamber, carrier group, etc.

I know this post is alittle off topic for the thread, but I thought I would pick your brain for some Colt info.
Thanks!