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Posted: 6/2/2006 8:41:36 AM EDT
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I'm looking for a dimension from a mil-spec flat top upper. How high is the top of the rail on a flat top upper above the bore centerline. I'm trying to troubleshoot a rail installation and I've seen many blueprints for lowers but can find anything for this dimension on the uppers. Thanks, SA |
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I measured an LMT, a Stag & a Rock River and made a drawing. I also measured a DPMS Low-Pro and I noted that it is 0.040" taller than the others. Something else to know is that there are 2 different heights of carry handles and front sight bases out there, mil-spec and commercial/A2. A2 front sight bases are 0.032" to 0.040" lower than "F" height military front sights on carbines. Rifles use A2 spec height front sight bases. Mil-spec rear sights are the proper height for an "F" height FSB. Commercial rear sights and carry handles can be found that are 0.032" to 0.040" shorter and would work properly on the DPMS when combined with an "F" height FSB. Proper zero can be obtained as long as you have the right combination of parts. There are 0.040" extra tall front sight posts available to help with this. If you end up with a short rear sight, you can shorten the front sight accordingly. 
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Brilliant job Randall and many thanks!!! Your results could explain alot as I've been fighting an install of an ARMS 58SM SIR midlength rail onto an Armalite M-15A4. The underbarrel yoke seems to fit around the barrel nut perfectly, yet the screw holes on the yoke cannot align with the holes in the mount. The yoke seems like it needs to go 30-40 mils further into the SIR rail to align, yet it is already bottomed out on the barrel nut.  Barring specific rail problems, the other possibility is that the Armalite upper rail is too high. From your numbers it looks like this latter condition may be the case.  I measured my barreled upper and came up with a value of 1.248" from bore centerline to the upper rail. Based on your mil-spec value of 1.215", this would put my Armalite about 0.033" too tall ... almost dead on for the yoke screw misalignment I am seeing.I have not gotten a clear answer from Armalite or ARMS on this, and two separate 58SM rails have had the same fit problem. It looks like I will have to dremel the yoke holes in the mount if I want to keep the SIR or else switch to a non-upper mounted rail, though these would likely not sit level with my upper either. Many thanks for your efforts Randall !! SA |
Just get another upper receiver that's to spec. Sell off the Armalite on the EE and you will only be about $40 or $50 behind and everything will be right in the end. |
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