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View Full Version : Need help troubleshooting my AR.


a1fabweld
09-17-2008, 8:26 PM
I have a CMMG 16" flattop mid length medcon upper that I ordered with a 13.5" Troy handguard & YHM rail mounted sights. The rest of the gun is a CMMG lower, 2 stage trigger, & Magpul stock but that doesn't matter here. If my mind isn't playing tricks on me, I believe I used to be able to align my iron sights with my Aimpoint Comp C3 (2x magnification) red dot. I took it to the range this past weekend & both iron sights & red dot were off. I recently installed a KNS crosshair sight post & adjusted it to approximately the same height as the original post. At the range, the KNS post needed to be adjusted way lower than there were threads to adjust it. I proceeded to dial in my Aimpoint & shot the rest of the day with it. When I got home, I switched back to the stock post that came with my front YHM sight. It too was too high. When I originally took it out, I counted approx 4 visible threads. I reinstalled it back to a very similar height. Today I removed the troy handguard (which I found to be very slightly loose), cleaned all around it, & reinstalled it. I did the same with both f&r sights. I used my American Defense Aimpoint mount (co-witness) to align the rail to the receiver by overlapping it on the two then tightened the handguard screws. I recentered the iron sights & aimpoint dot to be on the same plane. I fired a test shot from about 20' & it hit about 4" lower & 1" right than my target. I then adjusted the red dot to the p.o.i. & now the front sight post is so high that there is not enough room to adjust it down far enough to align with the red dot. I'm out of ideas. I know that YHM products are not very popular here, but the sights are very solid with no play. Any ideas? Thanks!!

Bimmer2
09-17-2008, 9:15 PM
Here's a wild guess: when the handguard was loose, the weight of the rifle resting on it leveraged it upwards, which put your front sights way too high, since they're mounted to it. You adjusted them all the way down, but ran out of travel...

Once you tightened your handguard, it returned the front sights to where they were originally. If your front sight is as high as it'll go and you're shooting 4" low, then you went in the wrong direction. Move the front sight DOWN to bring the p.o.i. UP.

Remember, too, at short ranges (like the 20' or 6m you mentioned), you have to account for the barrel (and p.o.i.) being a couple inches lower than the sight plane. When in doubt, zero at 50m.

Good luck,

Ben

ar15barrels
09-17-2008, 9:22 PM
Use a dedicated freefloat rail that does not rely on clamping on to a factory barrel nut like a Troy.

a1fabweld
09-17-2008, 10:00 PM
Does the 2x magnification affect the way the front sight is viewed? I can't remember when I first set up this rifle if the sights & red dot both matched p.o.i. at the same time of if I used them independently? I did put my Eotech from another rifle on this particular one tonight & it's dot lined up with the front sight in height almost right on. I wonder if the Aimpoint is off, the mount for it, or ME? Ben, the front post needs to go down but it's maxxed out of travel in that direction. I did overlook the spread of the sight plane vs. the barrel center at short distances. There are too many variables for my mind to process now! Ar15barrels, do you run into problems like this often with this style of handguard? Thanks fellas!

ar15barrels
09-17-2008, 10:04 PM
Does the 2x magnification affect the way the front sight is viewed?

I did overlook the spread of the sight plane vs. the barrel center at short distances.

Those are your problems.
Go to the range.
Zero either the irons OR the optic at 50yds.
Adjust the other to match AT 50YDS.

a1fabweld
09-17-2008, 10:32 PM
Those are your problems.
Go to the range.
Zero either the irons OR the optic at 50yds.
Adjust the other to match AT 50YDS.

Ok, but should the sights & red dot match simultaneously on p.o.i. with the 2x magnification added to the mix?

ar15barrels
09-17-2008, 10:44 PM
Ok, but should the sights & red dot match simultaneously on p.o.i. with the 2x magnification added to the mix?

the 2x introduces parallax.