Folks,

Have any of you had to add about +12 clicks to the right from "mechanical" zero @ 50yds?
Is this normal?

The upper is an LMT 16".

Thanks,
Jim

I have. I was a bit sad when I had to do the same with my Bushy upper. It still has room for adjustment so I'm not loosing sleep over it, but I hear that Bushmaster has a hard time indexing their FSBs.

CYBIN,

Thanks for the response.

The ARMS40 isn't permanent, I'm planning to use an LMT or LaRue BUIS.  Although I'm going the LMT/LaRue route, I am a bit concerned with the excessive windage on the ARMS40.

Jim

MY Bushmaster originally had that problem with a BMAS flip up rear sight and it was a visibly canted front sight tower. sent it back and they fixed it, no excessive windage.

Fast forward to a rebuild by a vendor here, adding a #40L and the sight is cranked all the way to the right, with front tower eyeball straight. I asked the vendor what he thought of this, and the best he could come up with is "sounds like a back up irons system to me".

CYBIN & AUT_BELLUM,

Did you guys have to make left or right windage adjustments?
If left, how many off center?
If right, how many off center?

I removed the #40 to check the FSB alignment with regards to the receiver, and all looks well (from the naked eye).

Jim

I am cranked all the way to the right. I inspected it and the sight sits slightly offset to the left, meaning the space on the upper above where the charging handle sits is slightly larger on the right side than the left. Enough to notice it by eye, but not much. I wish I still had the YHM sight to try out. My group is still slightly left of point of aim @ 50  yards. My optic is dead on and is sitting with it's triangle reticle perfectly on the front post. The rear sight looks annoying but it works, I guess. Probably a stack up of tolerences.  Put loctite on the hardware, mine loosened up and fell off never to be seen again. I had to spend 10 bucks and get new parts from ARMS.

AUT_BELLUM,

I am experiencing nearly the EXACT same anomaly as you are with the #40.  The BUIS seems "cocked" vertically to the left in relation to the flat-top & charing handle.  

I tried my detachable carry handle and as far as I can tell, lines up square with the front sight.

Bummer.

Jim

Hey Duke,

You're right. let me clarify. I meant to say, looking straight down at the sight base when flipped up it is slightly more left than right, not centered. Then I read your last post, and the sight is not sitting parallel to the top rail. I ran some feeler gauges, and it indeed isn't parallel. Looking from the rear, I can comfortably fit a .024 on the right side, and only a .017 on the left side. All these tolerances stacked up is causing the excessive windage. Definitley a bummer.

Wierd.  When I put my 40A on my upper (RRA receiver, Bushy barrel), it zeroed within three clicks of windage.  Must be lucky...

My 20" A4 upper matches just fine with the #40, but not with my LMT.

Oh well...

The #40 was just temporary for the LMT as it will reside on the 20" from now on.

Bummer...

AUT_BELLUM, I have about the same gap between my #40 and receiver.

Jim

Can you flip the BUIS 180 degrees?  Do you then have to crank it the same distance to the other side for the same zero?  I think that means canted FSB, right?

If you only have to crank the reversed BUIS part of the way back, then it would be an off-center BUIS.  Someone check my reasoning on this.....

Whenever I pull a barrel and the POI shifts radically (I've seen 12" at 100yds several times) I just have someone hold pressure on the FSB in the desired direction while re-torquing the barrel nut.  There is a little play to work with in the receiver slot / barrel extension pin interface.

Rich

I think mine may be a combination of three things. a slightly canted sight base(not readily noticible by eye, like the first time I experienced it), a sight base that is slightly off center, and is not sitting parallel to the upper receiver. all or some of these things stacked up is causing it. I'm not going to sweat it, it's zeroed and this isn't a match rifle. If TROY comes out with a small aperature defaulting flip up BUIS, I may use it on my next build, but will try it on this rifle first.

Excessive windage is an odd description, as something is not excessive if and when it is needed.
The military requirements have minimum adjustment ability, but no maximum. Windage alignment with the front sight is dependant where the front sight base is on the barrel, and how well the barrel is aligned to the front of the receiver. Since all rear sights are attached in some manner to the receiver, either a solid carry handle, or via a detachable carry handle or stand alone to a flat top receiver. The predominant method of attachment is to attach to the dovetail rails, and do not rest on the top surface. The dovetail angles are the control factor of where any sight is going to sit. The plus or minus in machining tolerances allowed by the mil spec, provide the manufacturers, the ability to give our troops a product that accepts variations from different military suppliers that work every time when within those certified specs. To put a feeler gauge under any sight to the top of a receiver, does not tell you if the sight is in spec or not. The top flat surface of the receiver is not where the sight is resting, therefore to check if something is correct or not, a dovetail gauge is required for proper inspection, just like the government does. If someone wants to see how much variatioss are in a weapon, just look at the variaions in just the barrels to the receivers, it's not three thousands of an inch like on a rear sight, try 20-30 as an average minimum from front to back of the barrels.  
An (excessive) lean to one side or another can’t show if a sight is in spec or not, the receiver angles may be the problem. Speaking mil spec, if the receiver angles are over size, then a sight will lean to the right, if the sight leans to the left it shows that the angles are under size. Take just these factors alone, barrel and or front sight housing placement, or rail dimensioned, you can easily realize that any rear sight is totally dependant on to which it is attached. Another thing to realize is that since the aperture is a round circle, it makes not one bit of difference in shooting performance as to lean in either direction as long as it is consistent.
When mixing after market receivers with military issue rear sights, you will find there are minor dimensional differences. Another thing you will find with any rear sight is that when you put the same rear sight on various receivers, you will very rarely find that you don't have to make a windage adjustment, even from the same weapons maker, do to all the variables, some of whish I have listed. Battle rifles get there windage knocked out in the rigors of extreme hard use, barrels stepped on, etc. Soldiers in the field have to re-zero if and when they have a chance or need, and they will tell you that there is no such thing as excessive windage. Colt and FNMI make detachable carry handles under government inspection. The fairly new Picatinny elevation style, plus ARMS and KAC make the fold down types under the same inspection criteria are certified as correct, for general issue.  
Good shootin, Jack

hmm... ARMS 40L here and I have a PRI flip-up front sight that is eyeball straight and an ARMS 40L mounted on an LMY flattop and I have to have the aperture adjusted nearly fully to the right to hit 1" left of POA at ~15 yds... any suggestions?

Quoted: hmm... ARMS 40L here and I have a PRI flip-up front sight that is eyeball straight and an ARMS 40L mounted on an LMY flattop and I have to have the aperture adjusted nearly fully to the right to hit 1" left of POA at ~15 yds... any suggestions?

See my reply in your thread addressing this. "Eyeball straight" on a front sight is not good enough. If it is just a couple of millimeters off to one side, that can make a big difference. My PRI flip-up looked straight, but was actually about 2 mm to the left of vertical; thus, my ARMS 40-L cranked almost all the way over. Once I got the PRI lined-up properly, the windage only needed to be moved a few clicks away from dead center.....