I recently bought a 6520 AR15-A2 Gov't Carbine and need to replace the front sight post. The previous owner had tried to turn it with pliers on the square post itself and bent it.

I would like to replace the unit with an original Colt front sight post but need some information on how to verify I am buying the correct one as it seems that there are two different sights of square posts used on M16A2 rifles/carbines. Which height would be correct for a 6520 rifle? Can I just buy a current production 6920 front sight post, or would this not work?

I have also thought about replacing it with an M16A1 round front sight post as this seems to be a finer aiming post - most people, however, seem to say that the fatter, square A2 post is superior. Why would this be the case?

The square post is better because a round post looks different depending where the light comes from.

@molon

@Molon

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



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).





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




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ETA: never mind, beaten by the man himself.

Great information - thank you!

Does anyone know if a 6520 carbine should have the tall or the short square post? I see that Brownells sells a .270 "short" post here:

https://www.brownells.com/gun-parts/rifle-parts/rifle-sights-parts/.270-post-a4-front-sight-steel-black/

but I do not know if it is correct for the 6520.

Colt typically uses the "tall" sight post on 9mm carbines.  The tall post can help obtain a RIBZ sighting scheme if the flange on the other sight posts rises above the sight well of the front sight base.  

...

Do the Colt posts have a specific width

I see - I will be on the hunt for a short post then. Would you be able to offer any information on which rifles/carbines the square SP64507 vs. the tapered SP64665 were used? It is hard to tell which post was originally on my rifle as it is so mangled up - which would be correct for a 1996 manufacture 6520 carbine?


Also, which type of post was used on the standard M16A2 service rifle (or were both used, with one being earlier production than the other)? Maybe it would be a good idea to hunt for a surplus M16A2 post if I cannot find a correct new manufactured one.

The short square front sight post (SP64507) is pretty much the standard. I could be wrong, but I'd believe that's what the 6520 came with. Now when Colt started using the same barrels from the 6520 on flat top uppers with the model 6720, Colt started using the taller 9mm FSPs on them due to the barrels having non "F" height FSBs. Though the first 6720 I ever got to inspect had the standard short square front sight post (SP64507) and it zeroed fine. However, most 6720s come with Colt's 9mm FSPs and one of my 2 examples actually needed it for zeroing.  

Edit: You can identify Colt's tapered FSP by the dimpled bottom on the threaded section of the sight post. See Specialized Armaments description. > Link.

Yeah, it's not a "short" post, it's the standard post. All variants are available from SAW as linked. I've got a "tall" for a build with a 10.3" barrel with "F" FSB on a 6520 A2 upper. Not sure if I'll need it. Also have one that's a bit taller than that from KNS.

So you are saying that the standard tall post was used on the 6520? I wonder if they shipped out early/late production pieces with different front sight posts.

Also what is SAW? There was a link to the tapered posed on Specialized Armament's website (never heard of the company though) and they seem to also have standard, tall M16A2 posts.

No. There is no "standard tall" post. The "standard" post and the "short" post are the same thing.
The standard (short) square post was used throughout the A2 line.
The tall square post was used with the A2 9mm SMG line.
I'm not familiar with where the tapered standard/short square post was used.

SAW is Specialized Armament, aka Specialized Armament Warehouse, aka SAW LE Sales. SAW is arguably the number 1 vendor when it comes to anything and everything Colt AR15/M16. Ken Elmore (owner) is widely recognized as the the premier Colt AR15/M16 gunsmith.
IMO, SAW doesn't get talked up more on this board due to some interactions with board members in the past.

OK - now I understand the picture completely. I will go ahead and order from SAW then.

Unrelatedly, I may be interested in replacing the rubberized front sling swivel on the rifle as it is somewhat chewed up. SAW has "old pattern" M16 sling swivels as well as the "current" pattern Colt swivels. Would you know if there is any difference between the M16 "old" pattern swivel and the swivel that comes on the 6520?

The swivel on my 6520 is definitely not the new production Colt type with the thick rubber. I am not sure if there were multiple variations of the early swivels though.

I would start a new post about the sling swivel. Lots of people here that should know about that.

Very insightful