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 dimple

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 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.

From page 264 of BR II there is one interesting, and impossible to verify, side note.

Eventually, when the existing stocks of large hammer and trigger pins are exhausted, Colt will be going back to the .155” diameter hammer and trigger pins in order to standardize on one single version of their parts.

thanks guys, been thinking on it for a couple of days and finally figured out what didn't make sense now I have to figure out why it doesn't make sense.

Apologies for the confusion Forest, the spec calls for bottom of receiver to bottom of slot but since I only had the bottom to top numbers from BR II I omitted the bottom to bottom numbers from the post. I've edited it to reflect the bottom to bottom numbers. Thanks for the new info, it's been added to the DB.

very interested in the carry handle/BIUS numbers, thanks Patrick, DB updated.

Quoted: I measured at the point indicated in your diagram - from the bottom of the 2nd slot to the bottom edge of the receiver.

Forest,

I made up the diagram to save having to type "bottom of receiver to bottom of slot" repeatedly. I never intendeed it to become a standard of sorts I don't think it matters where the uppers are measured but I don't have the print for them. IIRC, the height was a general number along the bottom of all the slots. It probably does make more sense to measure at one point though, if just for the sake of uniformity.

got gauge blocks or a micrometer?

Quoted: You never stop to amaze me! Thanks for the excellent info.

This is actually a compilation of several previous like themed threads. They're lost to the archive server now so I reposted with the new info (hopefully correct info) from BR II. You're welcome M4.

that's odd bc that came from Ken.

Quoted: The proposal was for a new "third" type trigger group, not a reversal to the old type.

OK, that kind of fits, Ken said that he had bought all of Colt's old style (big) parts and after they were sold that would be all of them.

Jetlag,

I got nada for carry handles and wouldn't know where to measure them. maybe from the bottom to the center of the windage screw hole.