I have a 20" pencil barrel M16A1 upper that I pulled the barrel off of, and upon reinstalling it shot about 3" lower than it used to.  The upper already shot low, but now I have run out of adjustment on the front sight post and it still shoots about 1" low at 25 meters.

Short of shaving down the front sight post is there another solution?  My thought was to pull the barrel and try reinstalling it?  The barrel nut was a little loose and out of alignment for the gas tube (that and need to replace port door spring) is why I removed it but I can only assume I may have overtightened it a bit and that is causing the probelm?

Sorry, I tried to look at the FAQ threads, but none of the links work anymore.

[eta] I had another person shoot his rifle and they had the exact same result of shooting low.  I loath to do any MAJOR surgery, as it shoots really nice and tight groups!

[eta]  BTW, it is a round 5 notch A1 front sight post and an A1 rear sight,.  Just wanted to mention that before someone says "adjust your A2 sight" or "replace with a shorter A2 front sight post"

Move out to 100 yards.

The sight axis is a few inches above the axis if the bore. If you adjust sights so your POI and POA are the same at 25 M, then you'll be "way high" at 100 yards.

I replaced the gas tube (and the port door spring) otherwise everything is exactly the same as before.

I was looking at the peep sight last night and noticed something odd- I always thought that there was no aperture with an "L" on it...then I looked at the rifle from the front and realized that the "L" is only visible on the peep when you have the muzzle pointed at you....um, is the A1 peep installed backwards???  The "taper" to each peep hole is visible when the weapon is shouldered, if that makes any sense.  Being that the "L" aperture is for longer range shooting I assuem using it will raise the POI some?

And I appear to have been using the non "L" peep.

From the top of the front sight post to its flange is .259", and from the top of the front sight post to the "flat" of the front sight base is .216" if that helps any.  

Tweak, thanks for your patience.  I realize some of these questions probably seem dumb, but when no one else around knows anything about AR's occasionally I find myslef stumped by an (apparently) simple problem!

Quoted:  My thought was to pull the barrel and try reinstalling it?

That's what I would do.  It sounds like the barrel simply did not get seated into the receiver the same as it was before.  

Good luck.

Height from top of post to base of flange is 0.335"

Dawg

Cool, hopefully between reinstalling the barrel and using the proper peep it will be on target.  Thanks for the help, sometimes you just need another person to look at the probelm to help figure it out.

Dawg

I did not remove the flash supressor.

I think it is highly unlikely I bent the barrel, as I never touched t beyond giving it a light tug to pull it out of the upper receiver and a light push to reinsert it.

I reinstalled the barrel yesterday, and I think I probably had overtorque the barrel nut- it took a HERCULEAN effort to get the barrel nut to turn at first, and I was simply unable to get it back to the barrel nut notch it was at before.  The first time I removed and reinstalled I had manged to get it a notch further than it originally had been, but this time I torqued it to the notch it had originally had been and it was still plenty snug.  

I am assuming that by tightening it back the the same torque/notch it had originally been at it will sight in to about the same spot it originally did.  I'll let you know when I get to shoot with it.

Is it shooting lower than where you aim or lower than where its supposed to be?  Remember the barrel is about 2 1/2 inches below the sight.  When you shoot from something like 25 or 50 yards the shots should hit about 2 1/2 inches below the point you aimed the rifle at.  If you sight the rifle in to hit "bulls eye" at 50 yards you will shoot high as you move from the target.  I have seen quite a few people sight an AR in at close range then say the bullet "rises" as they increase distance.  I know this is not your problem but it seems strange that the same barrel and upper wont sight in and you ran out of adjustment.