User Panel
Posted: 6/14/2005 11:44:04 PM EDT
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:
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. |
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As someone who manufactures an RAS, this is potentially valuable information. The only problem being the measurements taken to the bottom of the groove slot. That puts into play the 1913 slot depth tolerence. They should all be the same as the picatinny rail geometry dictates the slot depth. Alas, they are not.
In hindsight it would be far more useful to measure from the bottom to the top of the rail. I was looking for such info a few months ago as it would be valuable to manufacture a rail that in between so you are not at one end or another. We ended up going with the M4 rail height. I suppose in the scheme of things, you are talking about splitting hairs and in most cases 5 or even 10 thou out doesn't really remove any practical use. If you still have access to the receivers, it would be helpful to alot of us to get numbers from the bottom to the top Even so, the measurement should be from the bore centerline to the top rail to be of most use, but to get accurate measurements for that you must do them from print which nobody but the manufacturers themselves have access to. From the bottom of the receiver up stacks too many tolerences and doesn't give you quite the precision you are looking for. Oh well. Just my 2 cents from the peanut gallery. |
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Look closer, you'll see those numbers are included as well. Tweak's numbers are spot on. 1.840" +/- .005" from bottom of receiver to top of rail. This is one of the checks I do on every upper receiver I purchase.
It matters more now that more rail systems are designed with an uninterupted top rail, no gap. Bad receiver and the rails don't match up for height. If a rail system designer built off a bad receiver that could be a problem as well. Thank you Tweak for reposting this valuable information for those who missed it the first time around. I always love your threads like this one. |
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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.
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+1 on that. It's real information, not someone flapping their jaws with their opinion on something or some heresay story on something. We should have a "Real Information" forum for stuff like this. |
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How old is the info, and is it possible to get specs for the newer products that are out? Since you have already run it to this point, whats the possibility of getting new info?
I understand that people have old receivers, but most people are buying new flattops, and a list of them would be an actual asset. TIA, Stick |
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Stickman, the value of this information is in what upper receivers SHOULD be. Those are just examples of incorrect receivers. Almost every parts "manufacturer" (vendor of subcontractors parts really) including Bushmaster, RRA, DPMS, etc. has supplied incorrect height upper receivers at some point in their history.
Buy from someone who will take it back if you are not pleased and do the QC yourself. Personally I'd stick with Colt, CMT or LMT (in that order) for upper receivers. |
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It is an excellent post Tweak. I aint no engineer, but this was one of the major challenges with our current project and I wished I had seen the info sooner.
I would love to get my hands on the CAD tech data package for the M4. |
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Data Point:
LMT Flattop Upper: 1.844 from bottom of upper to top of rail 1.730 from bottom of upper to bottom of rail slot |
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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". |
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The old thread has some other's measurements. For your reference:
archive.ar15.com/forums/topic.html?b=3&f=12&t=209373 |
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Here's my submission:
RRA upper: 1.725" from bottom to bottom of slot. 1.853" from bottom to top of slot. I'm using a Tapco carry handle (looks like airsoft junk, but it was cheap) and with the stock A2 FSB, I have the post maxed out and it still shoots high. |
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FWIW...
Colt M4 1.725 1.843 Colt M4 1.731 1.841 RRA 1.741 1.850 MEGA 1.723 1.841 Olympic Arms 1.734 1.841 Olympic Arms 1.726 1.839 Bushmaster 1.728 1.840 |
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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.
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I measure carry handles from the bottom of the carry handle (the part that rests against the top of the receiver behind the rail slots) to the bottom of the slot cut out for the elevation drum. Not to tough to get a dimension here, and you are just measuring the height of the forging, and not parts adjustable in height. Maybe I need to post a pic?
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Where to you measure these to find out what you have? And can we assume that all Colts are "military" and all others are "commercial"? |
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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. |
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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 |
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Ok, so lets say I have an LMT flat top 10.5 upper. Would a GG&G MAD BUIS be the correct height to use in conjunction with the LMT fixed front sight? Would the carrying handle of a Colt LE6920 be the correct height?
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Never noticed that, good observation. Sounds to me like there are two dimensions then, with the sum of both being most important (as you already accounted for with you measurment idea). I would like to see measurements of both. |
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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. |
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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. |
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OK, did not write what I meant earlier, but you seemed to have got it. I measured from the bottom of the handle to the top platform that the rear sight base stops on. Then I measured from the bottom to the part of the base that the rear aperature rests.
DCH off a 1998 M16A4 with AA forge code: .832 carry handle forging 1.29 with rear sight Colt DCH forge code AB: .832 1.30 replacment Colt DCH from about 1995 forge A-6 with an ArmaLite NM rear sight: .832 1.27 DCH off a 1995 6721 with A-1 forge code in a circle: .832 carry handle forging 1.28 with rear sight There are some variables when you include the rear sight base, like is the first stop on the index wheel a half click up from bottom, or does the rear sight stop on the carry handle on the last click. |
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OMG - way too much information for my old gray brain.
I tried to follow all your thought patterns seeing if one of you can help those of us who have A3's that still shoot high with the replacement .040 from post. I installed the higher post and, even though I am now shooting on spot at 100 yds, the post is still turned too far out. I would like to have the post at least .075 higher so that it sits more solidly against the detent ball. My concern is that there might still be movement in he front post between shots. Can anyone out there help us with the front sight problems????? I love the challange of shooting with iron sights and as I doctor my handloads I want to make sure that I have addressed all the variables possible. Thank you all in advance. Sickticket |
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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) |
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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.
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For this measurement (Colt DCH) I got 1.098 with RSB on 6/3 |
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So Tweak, your saying that the reason there are two different height DCHs is because there are two different height flat top uppers? The taller milspec DCH goes with the shorter milspec flat top upper? The shorter not milspec DCH goes with the taller not milspec flat top upper? How and why do the taller "F" marked FSBs come into play? Are both the commercial and milspec flat top upper receivers when combined with their corresponding DCHs set at a taller height/different sight plane, then the rifles and carbines with the standard FSBs and upper receivers with fixed carry handles? If so, then why don't all flat top barreld uppers came with a taller "F" marked FSB, or at least that taller front sight post that Bushmaster sells? Also people are saying that Bushmaster uses milspec flat top upper receivers now, but they use the shorter "commercial" DCH don't they? |
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it is confusing, isn't it? I hope we get a definitive, short and sweet, simple conclusion out of this, as many of us mix and match components in our weapons most explanations have been overloaded with too much confusing fluff. we need .................... A=1 and B=2 and if you use B, you need C and if you use D you need E |
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From this thread
More numbers here |
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From this thread
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