Hey guys,
I was thinking about picking up a Colt 6520 and putting a flat-top upper on it. What will I need to lookout for? How about a Les Baer no forward assist upper?
I don’t see anything wrong with it.
You’ll very likely need a taller front sight post. Other than that you should be GTG.
Colt distinguishes between standard (A1 or A2?) and F-marked FSBs, don’t they?
-B
What do you mean? An F base is an F base. The “F” denotes the fact that the sight post shelf was cut a bit higher, for use with a flat top upper receiver.
See! I am glad I asked.
I mean Colt offers two different FSBs for flat-top and non-flat top uppers, correct?
-B
I believe you would either need a new front sight post or have someone put on a F FSB.
Yes, F marked, and not F marked.
Edit: Ah, I see what you meant, now. I thought you were saying there were different variations of “F” marked FSB’s. Sorry. I will be ok, really.
You can use the non-“F” front sight base… just get a .040 taller front sight post.
Greg,
I’ve been considering doing this to my 6520 lightweight Colt as well. FWIW Brownells currently has factory Colt flat top stripped uppers in stock. The only thing holding me back is I bought a complete CMMG pencil barrel mid length and don’t want to duplicate the carbine.
I have done this. The only pitfall is as stated that you may potentially need the taller post. I would order the post in case you need it, but try sighting it in with the stock post as there is some overlap between the two.
I avoided all of this by shaving down the FSB and installing a FF rail over it.
I’ve converted 6520s, and other fixed CH uppers. Some of them needed the post, some did not. In addition to that consideration, remember to get a ramped or unramped upper receiver that corresponds to your barrel extension.
A 6520 barrel with a railed upper, sporting a T1, is a light and very handy carbine.
That’s my idea as well. Rob, what hand guard did you use? I’m thinking DD Omega if I decide to go to 14.5" with perm FH because it’s a lightweight 2 piece or possibly stay with the magpul hand guards. Obviously if I don’t go with the perm FH I don’t have to be concerned with installation issues. I’m kid of taking my time on this one so that when it’s done I don’t decide to change the configuration after 3 months!
I did the swap on my 6520 using a CMT upper. The 50 yard zero worked fine with the existing front sight. I later added the DD Omega rail and a T1. This makes for a nice LW carbine.
My Colt 6520s have come with Colt’s tall front sight post (part #SP62447) and I’ve never had any problems zeroing them after converting to flat-top upper receivers when using genuine Colt M4 upper receivers.
Colt currently offers four different front sight posts for use on their AR-15 family of rifles and carbines. 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. All of the Colt square sight posts have a nominal diameter of approximately 0.070”. 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.
Colt 6520-F
The barrel from a 6520 should already have a taller front sight post in it from the factory.
All colt fixed handle carbines are like this dating back to the XM177 series.
I’ve done this swap before and, using a genuine Colt M4 upper receiver, had no issues whatsoever.
I did this about 5 years ago and was told by at least one person back then, quite emphatically, that it couldn’t be done and it wouldn’t work properly.
That carbine now has a couple 3 day classes and probably ~6,000 rounds through it with no issues.
What issues did they claim would occur? Catastrophic failure since it didn’t leave the factory that way? 