Posted: 12/29/2009 6:05:57 PM EDT
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I have a CMMG flat top rifle that I have recently put on a MI ff rail. I purchased a set of MBUS front and rear sights, went to the range to sight it all in and.....doesnt work so much.
I have to crank the front sight post way up. The sight post is sticking above the ears at the front. The rail is one level from the back of the reciever to the front of the rail. Is this fixable, have I mounted things wrong, am I just screwed? |
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Sounds like something is seriously out of alignment, most likely the rail tube or the rear sight isn't fully seated in the rail slot. Pics would help, as something has to be visibly off.
I'd strip the upper off, remove the bolt, place the upper in a cradle, and bore-sight it at an object at ~100 yds. If the alignment is off then I'd try: -Reseating the rear sight on the flat-top. -Remove both sights, then lay a straight-edge across the full length of the flat-top and rail to verify alignment. -Remove and reassemble FF rail, re-verifying barrel torque in the process. Verify barrel is centered in the rail tube. |
| I was having alot of problems with my MBUS. The windage on my rear sight was all the way to the right. Turned out that my FF tube wasn't perfectly clocked. Just thought I'd share my experience, So far they are working great. Take a very long, detailed look, at how your upper is assembled. |
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Simple fix, I think. If you have anti rotation screws, loosen them (I don't think you do, though). Tighten the jam nut into the rail, not the receiver. In other words the gap between the jam nut and rail will be between the jam nut and the receiver. This should straighten out the rail. It is probably low because it's hanging slightly without the pressure of the jam nut. If you have anti rotation screws tighten them last.
ETA: In your bottom picture if that is truly the way it looks you can see the rail is not centered top to bottom. It may be the photo angle though. Also, where are the screws that hold the trim ring in the front of the rail? |
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Quoted:
Simple fix, I think. If you have anti rotation screws, loosen them (I don't think you do, though). Tighten the jam nut into the rail, not the receiver. In other words the gap between the jam nut and rail will be between the jam nut and the receiver. This should straighten out the rail. It is probably low because it's hanging slightly without the pressure of the jam nut. If you have anti rotation screws tighten them last. ETA: In your bottom picture if that is truly the way it looks you can see the rail is not centered top to bottom. It may be the photo angle though. Also, where are the screws that hold the trim ring in the front of the rail? ^^ try this and let us know how it turns out |
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I'll triple the above posts.
I had a YHM rail on my old rifle, in the pictures it does not look like the lock nut is tightened correctly. Like the above posters said, make sure the forearm is threaded correctly and then tighten the lock nut against the rail and not the receiver. EDIT: Sorry noticed it's an MI rail, but the setup is similar, if not exactly the same as the YHM |