Formerly lost to the Archives, now resurrected for the FAQ

To summarize, there are at least two kinds of “Picatinny” flattop uppers floating around, not including the old OAI “Weaver” railed one. There are also two different height carry handles and two different height FSBs. These parts vary most notably in the height of the rail/sight over the bore.



Once upon a time, I compiled the following measurements from members of the old AR15-L.

Height from receiver bottom to slot bottom
UNK
UNK
1.721”
1.720”
1.721”
1.720”
1.724”
1.763”
1.757”
1.945”

Height from receiver bottom to slot top
1.840” +/- 0.005”
1.849” +/- 0.007”
1.844”
1.843”
1.842”
1.838”
1.840”
1.875”
1.871”
2.450”

Make
M4
C8A1/C7A1
BM flattop
Colt M4
Colt flattop
Colt M4
Colt M4
Armalite
OAI Picatinny 13 slots
OAI Weaver 8 slots

Contributor
BR II
BR II
Nick
Unipro
Rick from El Paso
Ron N
Ron N
Paul
Tweak
Tweak

Since taking these measurements, the makers may have changed their parts suppliers. That doesn’t change the fact that the uppers and tooling are out there. I think we’ve seen this most recently with RRA’s “our’s is the milspec” flattops. Old parts never go away they’re just remarketed as the next new thing.

As you can see, the proper height flattops are 1.840” +/- the AR15 standard of 0.005”.

According to Black Rifle II the thickness of the upper was increased after Dick Swan (ARMS) shoved a #2 pencil through the top of the original flattop design to demonstrate the thinness of the web there. There is an excellent picture of the pencil stuck in the ACR upper receiver. Let that be a lesson to some of you, sometimes a graphic example is the fastest way to prove your point, especially when dealing with “show me” types as so often found in the uniformed services.

I know the web there is thin as the first two piece flattops (made from milled down carry handle uppers with lengths of Weaver rail attached by screws) often strip out their screws. One manufacturer partially solved the problem by running the screws from the inside of the upper into the rail. This proved much more secure but is more time consuming and therefore expensive.

That’s now a thing of the past due to the advent of flattop forgings.

The problem with this theory of increasing the thickness to improve strength (and it’s one I’ve promoted) is that the suspect uppers listed above are roughly .030” thicker than the “good” ones.

That is one curious thing.

This is the second curious thing.

In BR II, the author (who I lost all faith in when I saw the Rule 3 violation in his author photo) states the following while discussing the C7A1 rifle and C8A1 carbine in use by CF.

From page 199:

In addition, the height of the MIL-STD-1913 rail from the bottom of the upper receiver is 1.835”- 1.845” and this the US-made M4/M4A1 and the M16A4 flat-top rifles and carbines must use a special front sight assembly (marked “F” on the left side), which sits correspondingly higher than the standard M16-type rifle front sight assembly. The height of the Canadian sight rail is 1.842”-1.856”, which ensures that the Diemaco add-on sights and handles maintain the sight axis as the original C7 fixed-handle receiver version. This allows the use of the same front sight assembly n the both the fixed sight and flat-top upper receivers, and permits Diemaco customers to upgrade their fleet of rifles to the newer flat-top version by means of a simple conversion.

He states correctly that 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 overall 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.


The sentence in his statement, which I have marked in red, makes less sense and I have considered ignoring it.

If the Colt uppers require (as he stated earlier in the book) a taller than standard (standard being 2.285" +/- 0.010") front sight base then the even taller CF uppers should require an even taller FSB.

These taller CF flattops are not the taller ones seen in the list of measurements shown above. The taller flattops there are 0.03” taller than USGI instead of the stated 0.009” of the CF uppers.

Have I lost anyone yet?

He also states that the CF 1913 rail has slight dimensional variances from the STANAG 1913 and has one extra (for 14) cross-slot on the rail. If true, I guess that is something to be aware of if you find a 14 slot flattop upper floating around. I don’t see how CF could, or would, want to vary from the STANAG but anything is possible. He does include several pictures of 14 slot uppers.

the print I saw, but didn't get a copy of, showed the measurement from the bottom of the receiver to the bottom of the cross slot that's why I went with that dim.

Some more data (measured off of a single receiver for each of the examples listed below):

Colt M4 upper
1.725
1.835

CMT (in the white)
1.727
1.837

For what it's worth, we have recently been sourcing some OEM parts from various GI subs and there are 2 heights of carry handles available (as Tweak states above).  One is usually called the "Military Version", and the other is usually called the "Shorter Commercial Version".

anyone else handy with calipers? might be good to add some dims for the detachable carry handles, say from the bottom of the handle (the flat) to the center of the windage screw hole? pinch the bottom to the bottom of the screw hole then add half the D of the hole? make sure the RSB is all the way down first of course.

that is easier to measure but I've seen detachable carry handles with thicker than normal RSBs onboard. the M4 uses a thinner (where the wind scale is) web under the RSB.

Quoted: And can we assume that all Colts are "military" and all others are "commercial"?

I'm betting that the DCHs that fit the milspec height flattops are "military" and the others are "commercial". Went round with this once at my old work when they first got into flattops. One reason BFI used to (?) warn that their uppers would't work with Colt DCHs w/o a taller post or maybe it was vice versa, either way, the screwy parts are out there. the .mil DCHs are taller than the commercial ones (if my mind is working correctly after being up all night) to make up for the shorter relative height of the .mil flattops.

so IIRC the worst combination would be a commercial flattop (the 1.76"/1.87" ones), standard FSB (2.285"), with a .mil DCH.

Quoted: Quoted: And can we assume that all Colts are "military" and all others are "commercial"? I'm betting that the DCHs that fit the milspec height flattops are "military" and the others are "commercial". Went round with this once at my old work when they first got into flattops. One reason BFI used to (?) warn that their uppers would't work with Colt DCHs w/o a taller post or maybe it was vice versa, either way, the screwy parts are out there. the .mil DCHs are taller than the commercial ones (if my mind is working correctly after being up all night) to make up for the shorter relative height of the .mil flattops. so IIRC the worst combination would be a commercial flattop (the 1.76"/1.87" ones), standard FSB (2.285"), with a .mil DCH.

Mongo,
As for who carries (retails) which, I am not sure.  But I would feel comfortable saying that Colt, CMT, and LMT are the "military" height versions (MilSpec Height) - which is taller.

Tweek,
As far as I know, right on all counts . . . .now get some sleep

FedDC,

from the numbers for LMT shown in this thread they appear to use the correct height upper, not surprisingly therefore any USGI spec rear sight should work with their upper. the concern is with the height of the FSB. knowing LMT it not one of the big concerns.

Ekie,

I did have the thickness floating around one of the HDs but it's not the one I have here with me. I agree it would be a good dimension to track. Being old school I have no DCHs to measure.

Paul,

Headed that way, gotta be up at 1115.

FedDC,
I think your set up will be good to go.
I have run the LMT 10.5 with a Colt handle, GGG BUIS, ARMS #40 and MI - all zero with no problems.

*

.

Tweak,
I have been told the difference in RSB housings is for an A2 upper vs a detachable carry handle.
I have not been able to confirm they with a caliper yet.
This is in reference to MIL-SPEC heights (thicknesses), and may or may not be true with commercial variations.



Edited to add:  For the RSB housings (both Colt), I am getting a .050 difference between the two.
(as a note, I did not disassemble the housings and it is a bit tough to get a caliper in there to measure)

the RSB used on the DCH should be thinner than the one used on the A2 but I have seen A2 thickness ones used on DCH's. IIRC, Chuck Santose had one turn up this way years ago.

Quoted: anyone else handy with calipers? might be good to add some dims for the detachable carry handles, say from the bottom of the handle (the flat) to the center of the windage screw hole? pinch the bottom to the bottom of the screw hole then add half the D of the hole? make sure the RSB is all the way down first of course.

For this measurement (Colt DCH) I got 1.098

with RSB on 6/3

From this thread

Quoted: Quoted: Quoted: If your going to use mil-spec rear sights (like a COLT carry handle or Matech BUIS) on a Bushmaster A3, you will need the .040" taller FSP... If you are using a Bushmaster carry handle on a Bushmaster A3 you do not need the .040" taller FSP... Shooters Den, None of my Bushy's have carry handles.  They all use military type BUISes, same with my buddies. In order for someone to 'need' the taller FSP the tolorances have to stack against him.  It happens, but most of the time there is plenty of adjustment in the front sight to allow zeroing.  Of course it helps when you zero at 50y rather than trying to zero at 25y (or even 25M). I have had different results... My COLT carry handle and MATECH BUIS elevation will not zero and the calibration is off on the MATECH when used with the Bushmaster A3 carbine, with the COLT carbine everything aligns up perfectly... I have recently discovered the reason why... Measuring the handles using a micrometer, the rear shelf of my COLT and Bushmaster carry handles (where the sight mounts to) are different heights, there is a .042" difference between the two...

More numbers here

From this thread

Quoted: Quoted: well i meant does it work no problems with the govt flat top upper. i have the BCM 20" Govt profile upper, it has the "F" FSB. I have no idea what the specs are for a CMT DCH... An easy way for you to find out is to go shoot it and see... If the barrel has a true MilSpec "F" marked FSB, then the worst case scenario would be, you'll need to lower the FSP... You can measure it to find out if it is a MilSpec or commercial DCH, that will give you an idea of where your at... You could use the specs listed below to get an idea of what type DCH you have... Commercial DCH: bottom of DCH to center of windage screw = 1.006" MilSpec DCH: bottom of DCH to center of windage screw = 1.051" Commercial DCH: bottom of DCH to aperture (Bottom inside of large hole) = 1.247" MilSpec DCH: bottom of DCH to aperture (Bottom inside of large hole) = 1.292" Which ever you come closest to when comparing your measurements to the above specs should tell all... If you are wanting to purchase a new DCH from CMT, ask the dealer your purchasing it from if it is MilSpec or Commercial... MilSpec is what you want if your FSB is "F" marked...