Hey guys i have a new colt 6520 and i replaced the a2 for a m4 flattie upper and i am wondering what rear back up sight to use if if i dont wont to replace the org. a2 front sight post/block. Thanks in advance for the advice guys!
Troy, Larue, or LMT
Troy
ARMS
GG&G
KAC
Larue
LMT
I’m not really sure what you’re asking. You should be able to use any rear sight.
Midwest Industries makes a good one too.
i thought that the front sight post was diff. height b/t a flat-top ar and an A2 config ar. I bel. there is a marked F height that is the standard for a flat top, but im not sure for an A2.
As far as I know, most rear sights are designed to line up with a standard A2 post. I have a flat top with MI SPLP’s and everything lines up perfectly.
I have a 6520 with a Colt M4 upper receiver swapped in. I have 4K+ rounds through it and use a Troy folding rear with no issues. It zeros fine at 50 yards without excessive elevation adjustment on the front post.
Sure but the rear sight stays the same. If you can’t get it to zero, just get one of the longer front sight posts and install it in the FSB.
The sight post is the same. The sight bases are different.
There are however aftermarket sight post that are taller.
Buy a Troy and try it. It worked fine for me, I didn’t have to buy a taller front post or doing any other funny stuff.
Bushmaster 0.040" taller front sight post.
KAC, ARMS, LaRue and GG&G BUISs have worked well for me on a Colt 6520-F.
Irrelevant…I misread one of the responses.
I’ve used the ARMS #40L and a Matech. Both worked just fine with an F marked FSB
Oddly enough, I had a Non-F front sight based Colt commando upper that I bought used. It had a tall Front Site installed. 
Colt actually makes several different height front sight posts. One of these days I’ll have to make some comparative measurements.
[b]SP62447 FRONT SIGHT POST (9MM) TALL STRAIGHT, SMG
SP61706 FRONT SIGHT POST (A1) - ROUND
SP64507 FRONT SIGHT POST (SHORT SQUARE - NO TAPER) RIFLE
SP64665 FRONT SIGHT POST (SHORT TAPERED) CARBINE[/b]
A Colt 6520 should already have a Colt height carbine post which makes the tip of the front sight the same as an F marked FSB.
…this might be the ? u guys cant answer on the black rifle platform.
Thanks guys, hot damn u guys know your ghost gats extremely well. I think Molon posted some outstanding pics. I’m an idiot after reading these great replies. I think i was thinking to much on the whole replacement of a gas or F marked height flip up front sight, rather than actual asking myself the damn the ? first…thus realizing that the mbuis is give or take some minor elev. adj. on the post, all rears should work. If i am correct on this then i have clearly at lease read and understand the great advice, and if not feel free to tell me im a donk b/c hell gents ive been slammin’ the wheels of the “short bus” since i was knee high to grasshopper>
Anyways as always my ? was FULLY answered w. pics, advice and pro tips right out of the sandbox/ and the AR industry. NOW if i could simply find one…ONE damn homo-sapien in one of the 3 local gun stores that could only do half as 10 min. of generated replies on here.
One of my Rifles has an ARMS #40 and another has a Matech. I think both of them are excellent, although the matech is being a bit optimistic with its elevation options.
for the money though would u roll on one of those great deals on those matechs that are new, but perhaps a demo lot of sort i guess. Ive seen them for b/t 40 and 60 bones, and i think they go for ard. a hundo new. I like the ultra low profile, b.c my intent is to pop a trijicon 1-4 w. amer. def. recon mnt. Now if i can just justify the jaw-dropping 700 or 800 hundo for the optic setup, and force myself not to think of the complete upper i could be close to getting for the same $.
O_o
I can only assume hundo means money. Your post was a bit hard to read there to be honest with you. I paid 60 bucks for my matech from a guy on TOS. It is a good site and I like it. My I just wish it had a larger aperture.