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Posted: 7/21/2009 12:01:00 PM EDT
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I need some help from you guys. I just took my brand spanking new CMMG 16” mid-length flat top to the range. F marked FSB. BUIS is ARMS 40L. Using the US Army 25 Meter Zeroing Target at 25 meters. Aperture on BUIS was centered when I started the sighting in. 1st 3 shot group was low (2 clicks) and left (5 clicks) as were the subsequent 3 groups. I made the appropriate or what I thought to be appropriate adjustments, and I am still left. Elevation is good, but still left even after 6 more clicks to the right for a total of 11 clicks right. Any ideas? Bad BUIS? FSB canted? How do I skin this cat?
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Quoted:
I need some help from you guys. I just took my brand spanking new CMMG 16” mid-length flat top to the range. F marked FSB. BUIS is ARMS 40L. Using the US Army 25 Meter Zeroing Target at 25 meters. Aperture on BUIS was centered when I started the sighting in. 1st 3 shot group was low (2 clicks) and left (5 clicks) as were the subsequent 3 groups. I made the appropriate or what I thought to be appropriate adjustments, and I am still left. Elevation is good, but still left even after 6 more clicks to the right for a total of 11 clicks right. Any ideas? Bad BUIS? FSB canted? How do I skin this cat? Send it back to CMMG. Remove the BUIS and take the upper off the rifle. Hold the upper at arm's length and sight down the rail. Is the FSB canted from visual inspection? I've got one of my uppers back at the manufacturer for the same thing but my ARMS 40L had to use 20 clicks to the right to sight in and the FSB is canted ~ 2-3 degrees from top dead center. I asked the question last week and did some searching and found people quoted between 17-22 clicks was too much, but nobody could back it up with documentation. There is going to be some variation on how many clicks from mechanical zero to properly sight in a rifle, but at some point there has to be some sort of spec to determine how much is too much. With your windage of 11 clicks x .75 MOA/click = 8.25 MOA which is ~ 8 1/4" at 100 yds. 20 clicks x .75MOA/click = 15MOA which is ~ 15" at 100 yds. Its not an unheard of thing with any manufacturer but CMMG should make you happy if they know about the problem. ETA the archives seem to show quite a few posts over the years of members having the exact same problem as you and me with the ARMS 40 series sights and an array of manufacturer's uppers. They seem to clamp down and cant the sight to one side or the other. |
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Quoted:
Bloodsport I was concerned that it might be a canted FSB. I'll separate the two halves and give it a look. If I turn BUIS anymore to the right the aperture is going to be rubbing on the release lever that holds the sight down. What a bummer!  Before doing anything, try something like a carry handle or other brand sight because the ARMS do have a reputation for canting caused by several different reasons. I'm starting to suspect my canted FSB was a minor problem and my major problem was an ARMS causing problems. |
| Could also be a mis indexed barrel and in that case I just soda can thin aluminum and cut a half moon shape to fit halfway around the indexing pin then reset in the upper,Nice snug and no barrel movement...I garuntee youll be zeroed within 3 to 5 clicks windage tops.Military acceptance standards I think is 10 clicks and ive also heard 12.Put on the carry handle and turn the upper upside down and see if it sits flush on the FSBs ears and the rear sights ears..that will give a hint to if its FSB canting or not. |
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I have an ARMS 40 rear sight that cants to the side when tightening it down on a rail. I resolved the problem by purchasing a cheap set of metal feeler gauges and finding one that just barely fits between the top of the rail and the bottom of the sight. I snipped it down to a nice rectangle and now the sight sits nice and square atop my rail.
The same fix may work for you. Stop by at your local auto parts store or hardware store and pick up a cheap set of metal feeler gauges. |
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