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Posted: 9/2/2004 8:00:51 AM EDT
Got my new LMT upper last week. Had a chance to briefly take it to the range
before I started getting rained on heavily and had to bug out for work.

Off a rest, I was getting 1/2'' groups at 25 yards. However they were all 2.5-3 inches to the left. Didn't have a chance to adjust for windage that day, as I was running late, so I obviously need to take it out again and see what's up.

I have heard some say (including Bushmaster themselves) that 12 clicks in either direction is excessive. I have also heard some say that all the way to the left or right is okay as long as it gets you zero'd.

What do YOU guys think?

Here's what LMT told me via email:


I couldn't find any references in the TM's as to what the .mil considers excessive so I'm not sure what to think. It seems to me that if the upper was made correctly, it should be at least CLOSE to mechanical zero.

By my calculations (which may be wrong) at 25 yards 1 click will give you 1/8th of an inch, in which case I'd have to go probably 16-20 clicks left to get zero.

I had a Bushy upper that required 14 clicks left to zero, and they REPLACED the barrel for me. It had a canted FSB relative to the index pin.

If I had the tools I'd pull the barrel I'd give it a look see.

Link Posted: 9/2/2004 10:17:04 PM EDT
[Last Edit: 9/3/2004 1:58:54 AM EDT by Tweak]
Link Posted: 9/3/2004 1:02:59 AM EDT
[Last Edit: 9/3/2004 1:04:45 AM EDT by JLM]
Tweak, its a 16 inch upper with a carry handle. What do you suggest I do here? Nobody seems to think its a big deal.

Isn't it 1/8th of an inch per click at 25 METERS?

Thanks man :)

Link Posted: 9/3/2004 1:27:14 AM EDT
Wait ;)


Are you talking about 3.4.6?

3.4.6 Tarqetinq and accuracy. A series of 10 rounds fired
from each carbine at a range of 91.4 meters shall be within the
extreme spread and targeting area (heavy outline) specified in
Figure I when the front and rear sights are set as follows. The
normal rear sight peep (sight rotated fully rearward) shall be
used with the rear sight set centrally in the slot for windage
within plus or minus twelve (12) clicks. The top edge of the
front sight post flange shall be set flush to .030 inch below the
bottom surface of the front sight slot. Ammunition

12 clicks @100 yards, how many clicks does that translate to @ 25 yards? If I'm doing the math correctly, the max acceptable @25 yards would (6 inches @100 for 1/2 MOA clicks) or 1.5 inches at 25 yards? O wait, that would STILL be 12 clicks. In a no wind situation, clicks are clicks.


Link Posted: 9/3/2004 2:19:37 AM EDT
[Last Edit: 9/3/2004 4:50:47 AM EDT by Tweak]
Link Posted: 9/3/2004 4:01:11 AM EDT

However, if the 16" and 12 clicks are true, all my previous calcs that have been around for years are now in error and I'm not now in the mood to redo them. Oddly, the M4A1 spec is dated only 11 days from the M4 spec.

See, I got to thinking about that after I posted, and I looked carefully at that target in the link.

If I am READING it right, you get 12 clicks to be WITHIN that 16 inch zone on the target, in which case you would still have 8 inches of windage that you would have to account for to reach zero.

In other words you could be shooting 8 inches right at 12 clicks and that would be considered acceptable. Or 8 inches to the left at 12 clicks.

Link Posted: 9/3/2004 4:38:20 AM EDT
Link Posted: 9/4/2004 5:28:55 AM EDT
[Last Edit: 9/4/2004 5:29:14 AM EDT by JLM]
Tweak, should my rear sight look like this:

Link Posted: 9/4/2004 6:55:52 AM EDT

Link Posted: 9/4/2004 10:41:17 AM EDT
Did LMT supply the carry handle?

How would you determine whether the "excess" windage offset was due to some FSB cant , vs. a stack up of tolerances in the flat top rail, carry handle base clamps, and aperture itself?

Link Posted: 9/4/2004 10:42:09 AM EDT
I think it looks like your rear sight it leaning towards the left (my right)........
Link Posted: 9/4/2004 11:36:18 AM EDT
[Last Edit: 9/4/2004 11:39:41 AM EDT by Tweak]
Link Posted: 9/4/2004 6:21:10 PM EDT
[Last Edit: 9/4/2004 6:21:52 PM EDT by JLM]

Run the rear sight base up and see if the offset corrects itself. If so the threads on the rear sight base are probably incorrect.

Well well well, when I run up to max elevation two things happen:

1. no more side to side play
2. it straightens up the rear base sides relative to the channels on the carry handle.

So, if its thread incorrectly, like it looks, what's the next step?

Link Posted: 9/4/2004 10:23:12 PM EDT
Link Posted: 9/4/2004 10:49:54 PM EDT
So I need a new RSB then?

Link Posted: 9/5/2004 12:15:55 AM EDT
Link Posted: 9/5/2004 2:54:31 AM EDT
Thanks for all your help.

What's the easiest way to remove the RSB for inspection?

Link Posted: 9/5/2004 11:41:39 AM EDT
Link Posted: 9/5/2004 11:53:00 AM EDT
Time to break out the JB weld to build up the contact rail on the upper receiver, and then re-index the barrel to allow the rear sight to be centered zeroed.

It's that or you need to send the whole mess back to be corrected by the suppliers.
Link Posted: 9/5/2004 7:15:07 PM EDT

Time to break out the JB weld to build up the contact rail on the upper receiver

Are you thinking the flattop is perhaps not flat?

I just a barrel replaced by BM because it wasn't indexed right, and required 16 clicks left to zero. They were more than happy to do it. It took that upper and the LMT upper out today to do a side by side
comparison and got......

BM: so close I didn't even bother to dial in windage. Pretty much dead on

It's that or you need to send the whole mess back to be corrected by the suppliers

I'm going to ask for a RMA, even thou they don't seem to think its a problem.

Link Posted: 9/5/2004 10:16:01 PM EDT

Originally Posted By JLM:
Time to break out the JB weld to build up the contact rail on the upper receiver

Are you thinking the flattop is perhaps not flat?

No to build up the leading edge on the housing to take some of the rear sight cant out of the housing on the upper. Looks like either the upper housing sight slot was milled too far forward, or the front of the sight housing was milled to far back.

As Tweak note, see worse, but doesn't mean that you have to live with it.

By the way, did the upper and sights come as a unit, or was the sights added on later?
Link Posted: 9/5/2004 10:30:05 PM EDT
[Last Edit: 9/5/2004 10:30:54 PM EDT by JLM]
Dano, its' the carry handle off my BM upper. No windage issues on that upper now that I have a properly indexed barrel. I used the same handle on both uppers today at the range. BM was fine, LMT was not.

No to build up the leading edge on the housing to take some of the rear sight cant out of the housing on the upper. Looks like either the upper housing sight slot was milled too far forward, or the front of the sight housing was milled to far back.

Are you talking about my carry handle here?


Link Posted: 9/5/2004 11:17:30 PM EDT
[Last Edit: 9/5/2004 11:28:08 PM EDT by Tweak]
Link Posted: 9/7/2004 8:18:54 AM EDT
Tweak my man,

I got an interesting email from Bushmaster this morning:

All A2 and A3 rear sight bases are designed with a spring loaded detent and spring which rotates the sight base slightly to the left. They are like this so that the sight base will not bind when changing elevation and returns to the same sight plane after adjusting the elevation. Also if you look straight down at the flip aperature, you will see that it has been machined to offset the counter-clockwise turn of the rear sight assembly

Hrm, is this true? If I'm reading this correctly, they are saying that allthou the RSB will be twisted to the
left slightly by design, the actually aperture itself will NOT be. Mine IS pointing left.

Link Posted: 9/7/2004 12:10:22 PM EDT
The large and small peep apertures are off set to correct for the flip when going between the two (same centering in the housing), but the overall sight its self aligns with the sides of the sight housing.

As for the slight twisting of the sight housing on the upper (or carry handle), again it's normal. On some rigs that I run standard sights, the twisted sight housing/peep doesn't bother me too much. But on rigs that I am going to run a hooded rear sight, any twist of the housing off sets down the hood and really gets on my last nerve. On these DCM type rigs, its best to just change the detent spring and ball out to a two-rod tracking system to hold the hosing rock solid and allow more precise repeatable adjustments. On combat rifles, you want the rear sight housing to be able to take a hit and still self-center/function. But, this still puts us back at the twisted rear sight in the upper housing socket on your rifle.

On some uppers, the twist is very light, and on some others, quite twisted. As we have said before, we have seen worse than yours in regards to being twisted. Now if you wanted to take some of the twist out of the rear sight to housing, then the easy way is to just build up the guiding track of the upper with JB weld (the other side than the detent ball tracks).

To go the next step in your questions, in regards to the bushy carry handle center zeroing on the bushy it came off of, but not the new LMT upper, then there maybe a difference between the mounting surfaces from the bushy upper/the LMT upper and the carry handle (handle canted over to one side on either the Bushmasters or the LMT).

Now to make this really simple, had you purchased a LMT carry handle with the upper, then they would have confirmed that the sights were in line with the bore line. To go one-step father, had you bought one of their sighted carry handles, and the barrel needed to be re-index to center zero the rear sight, if sure that they would have done this without question. But since you are using a different brand of carry handle that may be slightly different spec than their own, they can only bore sight the front tower off the centerline of the flat top receiver. Final tweaking of the barrel to index the FSB to the "other than their own" carry handle will need to be done by you.

As for your Bushmaster carry handle indexing with a Bushmaster rifle, it's there own parts and should do so. Again, the fact that Bushmaster handle does not rear sight center index to the LMT upper just means that each manufacture mills to slightly different specs (+/-) and although parts drop on, tweaking may be required for fine tuning.

The real question is whether the LMT upper will center rear sight zero with a LMT carry handle, and the only way that you will find that out is to buy one of there carry handles to test. If the LMT upper with a LMT carry handle does take more than 13 clicks to off center to zero, then it's time to send them the upper back to have them correct the FSB index problem.

Bluntly, the

is the reason for the 12 clicks off center allowed by the military. No manufacture/supplier that I am aware of believes that as long as the peeps are not touching the sides of the sight housing everything is OK. Bushmaster sent a few rifles out with the barrels indexed wrong, but after having to spend money and time re-working the rifles to correct the problem, found it just simple to double check and make sure that the index was correct before leaving the first time out. Now it they can just overcome loose carrier keys, and purple parts, there will be Joy in AR Bushmaster-land.
Link Posted: 9/7/2004 2:59:41 PM EDT
Link Posted: 9/7/2004 9:55:37 PM EDT
I'll take a look at that, thanks :)

they can only bore sight the front tower off the centerline of the flat top receiver

Indeed, I'm guessing that's what they did, because it was a stripped upper that I purchased. Allthou
on the QC tag that was hanging off the front sight, they have to box for "REAR SIGHT FIT/FUNCTION" checked for some reason. Hard to say exactly what that means really. Did they put one of THEIR handles on it and bore sight it? I don't know one way or another. I do know that I finally talked them into issuing me a RMA number.

As I myself look down the centerline of the flattop, its fairly clear to me that the FSP is canted to the left.

You guys have been most helpful, thanks :)

I think I'm going to invest in some barreling tools here in the near future, it sounds like they would be well worth the money.

Link Posted: 10/4/2004 11:08:32 AM EDT
my M1S a3 carbine has to be turned 3 full turns RIGHT on the NM rear in order to zero at 25yds. :(
Link Posted: 10/5/2004 11:50:24 PM EDT

Good news. I'm getting a whole new upper from LMT. Supposedly it shipped out today.

I was doing some reading over at TF, and found your post about reliability testing. So, the first thing I am going to do is load up 4 30 rounders, and dump them as fast as I can. If the gun doesn't run right, then it will go back AGAIN until they get it right.

Thanks for your help again :)

Link Posted: 10/6/2004 12:05:50 AM EDT
Link Posted: 10/6/2004 1:33:16 AM EDT
Tweak, I just thought of something: I should probably run your 'mag dump' test with an H Buffer, in order to get a valid 'result' right?

Link Posted: 10/6/2004 1:42:40 AM EDT
Link Posted: 10/6/2004 1:47:22 AM EDT
But not for the 16's eh?

Link Posted: 10/6/2004 2:44:11 AM EDT
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