I just finished building my M4-gery and put on a SIR and an ARMS flip-up rear sight. I took it out to the range today and started with sighting in the iron sights (I also pit an Aimpoint ML2 on). Starting at 25 yards with the sight mark in the center. I was almost 8 inches left! I dialed kept moving the sight right, firing 3 shots and moving some more. That rear sight still shoots almost an inch and a half left, and I'm almost out of right adjustments. (I had no problem with the elevation with moving the front sight.)I switched to the Aimpoint and nailed zero with very little adjustments. Does anybody have any idea what I screwed up with the rear sight that the windage is so far off??? HELP!

It's not your rear sight, its your front.  Specifically, either the front sight tower/gas block is not squarely on the barrel and is canted to the left, or the barrel has been tweaked a bit to the left when the barrel nut was torqued on.  Did you install the barrel to the upper yourself?

Because the Aimpoint is mounted to the receiver and isn't dependent on the front sight, it doesn't have a problem.

The barrel was installed by Bushmaster on an M4 upper. I have not even thought about touching the barrel. How can I check to see if the front sight is on straight? If this is the problem, how can I fix it??

Is there a way to tell if the sight is twisted. I looked down the plane of the rifle from both the muzzle and butt. The sight assembly looks straight to me. Any other more precise way to tell than just eyeballing it?

OH NO! You didnt do an Al Gore....

[img]http://www.alamanceind.com/gore_gun.jpg[/img]

shootusp
I have had 2 bushmasters (SOLD ONE)still have one, I put an ARMS #38std on one and an ARMS#40 on the other and both were hitting exactly the same place as yours!! I loosened the barrel nut on each and repositioned the barrel, then using a barrel vise and a receiver vise held it in position to tighten. It helped some but still was off about 3-4 inches, my site is set almost( all but 3 clicks) to the right. I hate it because it doesnt give any real correction for windage. My best friend has an ARMS #38EX on an M4 Bushy clone as well same problem but he didnt move his, he just aims a little right.
In addition you have to have the extra long site post installed on the Bushy site base because it is not compatable with the ARMS height requirements. This leaves post sticking up as high as ears on site housing. You may try changing out site base. It may help. Oh yea sent the one back to Bushy, they basically sent it back and said nothing is wrong, you are imagining this!!!
I sold one because of this and I wanted to try an Armlaite. Do Not have same issue on it And Love it!!!!
good luck
cp

Eagle-

I don't think the wife will be happy if I sold this rifle and went to another....She might use it on me (she'll compensate for the off sights). Thanks for the reply though, at least I'm not alone on this. I guess I'll just stick with the ML2 and compensate with the sight. Have you heard about anybody trying to shim one side of the rear sight? I just thought about that. Do you think it could work?

TO ALL,
A couple of years ago I had a verry expensive Bushmaster upper built for me by Bushmaster. This upper had the fluted barrel and all the goodies. Had the same problem, with point of impact being too far out for the sights. Sold the upper to a friend who thought he could fix it.
WILL NEVER DO BUSINESS WITH BUSHMASTER AGAIN!!! Customer service on this was crap also!
Went tith a Colt M-4 upper and couldn't be happier. The problem with your sights being off is that it's from Bushmaster, plain and simple. [:(!]

Same thing happened to me with my $1500.00 ZM LR-300 upper. It's a common problem that is easy to fix. If you don't have the tools give bushmaster a call. You shouldn't have to live with it.

Here, perhaps reading this will help, FWIW, my ARMS# 40 sets far right on one of my M4s, but it's zeroed so what does it matter...., nada to me.

[b][red]Gunsmithing TIPS from Chris[/b]: Barreling Uppers...
Installing a barrel is really pretty simple. Solving some of the problems you can run into isn't. [b]Probably the most common problem is excessive windage present after the installation[/b]. The rear sight will be cranked all the way to one side. This is one time the manual doesn't help much, most military armorer learn this trick early. [b]Here's how to fix it[/b]. Clamp the barrel in a vise, using barrel blocks. Line up the front sight carefully on a vertical line, just like the book says. First tighten up the barrel nut - hand tight, not torque to prevent damage to receiver. Now, look at your receiver. Chances are it's leaning off to one side -when compared to the front sight. Using a non-marring hammer, hit the side of the carrying handle, as near to the front as you can. This will rotate the receiver slightly to one side or the other. Here's the trick, hit on the side that the rear aperture is furthest away from. In other words, try to move the receiver towards the rear sight. Be careful, you don't have to kill it, just a firm tap will usually do the trick. If the barrel is straight, aligning the receiver forging with the front sight forging will usually put the windage adjustment right in the middle, and it takes about a minute to do it. Simple! One other trick we should mention is for when you are torquing on a barrel nut and everything locks up. A squeaking noise, then it's like it's welded right there. The surest way to break something is to keep trying to loosen it. This trick is so simple, we couldn't believe it. Put the whole assembly in the freezer, and leave it overnight. This gets some differential expansion working for you. Pull it out the next day, and it will almost always come loose. We recommend using an action block, like the one we sell, and clamping on the receiver to prevent breaking the index pin when doing this. A really good wrench, like the heavy duty one we sell, will prevent ruining the barrel nut.[/red]

Mike

I've heard this more than once, "The ARMS #40/#38 sight is the wrong height with the stock Bushmaster front sight".  I've had both on Bushmaster uppers and never needed an extra high front sight.  Since Bushmaster builds to mil-spec (I know, they aren't full auto and not pursuant to a DOD contract, therefore they aren't "mil-spec"), and Colt does not, I find it strange that ARMS would build their sights to a height that is not compatible with every other mfg and with the military.

Bushmaster do not sell to the US military, period. They do not go thru US gov't inspection, period. COLT AND FNMI do sell M16 familly weapons to the US military, period. Colt and FMNI do go thru gov't inspection, Period. Bushmaster and other after market AR makers do not have the gov't prints, they make by reverse engineering, and that's why there are interchange problems of components sometimes. Not opinion or hearsay, and all can be found on any of the gov't contract award announcements board, Busmaster is not ever there, period. That means that only Colt and FMNI are to mil spec as far as the gov't is concerned, Period. Colt, FMNI, ARMS, and KAC make rear sights that go thru gov't inspection for issue to the Gov't M16 familly of weapons, it is fact that if the rear sights are not to gov't spec, the gov't wouldn't by them by the millions thru the many years. Bushmaster make good guns, but they are not to the gov't (certified) dimentions, actually thier front sight housing is an average of .030 lower than mil-spec. That is why you sometimes, on some Bushmasters have to get an extended front sight post to work with a mil-spec folding rear sight made by ARMS and KAC.
Hope you take the time to verify these clarification, not opinion.
Good Shootin, Jack

Excessive windage is usually caused by a front sight base not being "plumb" or "at top dead center" on the barrel.  I have seen scores of older M16A1's with this problem and in some cases could correct some of the windage by re=positioning the barrel alignment pin in the UR notch, but these were 6,000 round ++ rifles and there was always wear apparent in the UR notch.
When I finally got to look at government demenional drawings of these parts, it was obvious that a new notch was supposed to be .125" wide, and the barrel alignment pin also .125" in diamater.  So there should not be very much rotational movement of the pin in the notch.
Inspecting for excessive windage during USMC Limited Technical Inspections (LTI's) was a "learned skill".  We usually set the rifle on the deck standing on its buttplate between our feet.  Then we looked straight down the barrel and sights.  In this way it was pretty easy to see any "cant" of the front sight base, especially if you had a rifle right next to it that was plumb (i.e., no excessive windage).
So your cant is most likely a fixturing problem at manufacturing when the tapered pin holes are drilled and the pins then set.  Each manufacture uses different fixtures and obviously different machinist to do the work.  Now between all these variables, the G.I. Spec calls for the rear sight to be like 4 or 5 clicks of its mechanical center when zeroed with 10 rounds at 100 yards.  Now the target they use is pretty big (4 or 5 more clicks wide lets say if you were hitting in the black at 9 o'clock), but this was still proof positive that everything was manufactured and aligned accordingly.
In a case like yours, the manufacturer should replace the barrel.