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Posted: 12/14/2008 6:02:25 PM EST
I just bought a brand new mid-length rifle kit from Del-ton. I assembled the lower parts on a Double Star receiver. It is a pretty plain rifle for my first attept to build a lower.

It is dead nuts 25 yard battle zero, but I had to use all the windage on the rear sight and all the elevation on the front. There is nothing left. The rear sight is max deflected to the left and the front sight is at the lowest possible position. When I look down the barrel and sight, I think I am seeing things. It looks like the barrel is not aligned with the sights even tough the bullets impact where the sights are aimed. When I look down the barrel on a plumb line, the sights are off to the right.

I pass the rifle around and everyone agrees that sights are zero at 25 yards.

WTF?
Link Posted: 12/14/2008 7:04:16 PM EST
[Last Edit: 12/14/2008 7:04:31 PM EST by Dano523]
Link Posted: 12/15/2008 4:04:34 AM EST
I just went through this with a rifle kit that my son and I put together. His rifle (a 16" flat top with DPMS BUIS) shot 4" right and 4" low at 25 meters - that's 16 MOA off. We also used up all of our windage on the rear sight.

Rather than send it back, we diecided to fix it ourselves. We had invested in all of the tools we needed, so we just took it apart, pulled the barrel, filed the barrel locating notch in the receiver an appropriate amount, then re-assembled it. It now shoots pretty much to POA.

He did contact Del-Ton about it by e-mail, but we were not encouraged by their response - they just questioned how long he had been shooting a rifle! BTW, both he and I shot the rifle for sight in, and it hit to the same point for both of us. We double-checked it by using both the BUIS and a cheap Barska holo-sight - all results were consistent with the front sight being mis-aligned.

I have further notes regarding this in this thread.

I bet we're going to be seeing a lot of this in the next few months. In Del-Ton's defense, I didn't see anything wrong in their assembly - the locating pin was tight in the notch, and the notch didn't seem to be deformed in any way. I have no way of confirming this, but I bet that they buy their barrels complete with the front sight base already attached by the barrel manufacturer, and this guy (Wilson, in our case) is letting his QC slip under the demands that the market is placing on him. The front sight base has to be accurately located relative to the barrel extension locating pin, and I bet that they're getting a little careless in their rush to get barrels out the door.

Anyway, we just decided to fix it ourselves. Most AR work is pretty much just a plumbing job, anyway.

Link Posted: 12/15/2008 3:57:29 PM EST
[Last Edit: 12/15/2008 3:58:47 PM EST by kilo_juliet]
Originally Posted By tpelle:


filed the barrel locating notch in the receiver an appropriate amount, then re-assembled it. It now shoots pretty much to POA.




Will this change POA for the next barrel?

How much did you need to file off?
Link Posted: 12/15/2008 4:13:05 PM EST
Must have been put together by some ex Bushmaster employees
Link Posted: 12/15/2008 4:24:20 PM EST
Link Posted: 12/16/2008 1:06:04 AM EST
[Last Edit: 12/16/2008 1:09:02 AM EST by tpelle]
Originally Posted By Dano523:
Originally Posted By kilo_juliet:
Originally Posted By tpelle:


filed the barrel locating notch in the receiver an appropriate amount, then re-assembled it. It now shoots pretty much to POA.




Will this change POA for the next barrel?

How much did you need to file off?


Since sending the upper back appears to not be the option, lets just deal with the problem instead.

Take the upper off the lower.

Take a look at the direction that the rear sight has to be moved back to center, this is the direction that the barrel needs to be slipped in the upper barrel socket.

Now lay the upper in you lap with the receiver on one leg, and the barrel just behind the FSB across your other leg.

Using a leather mallet (will not mar the finish) and holding onto the receiver only, make sharp blows to the FSB just above the barrel to slip the entire barrel in the upper barrel socket. The FSB is taper pinned to the barrel, so it will not rotate, but since the barrel is retained in the upper barrel socket via tension alone, the barrel is going to slip in the receiver.

If you do this at the range, then you can slip the entire barrel in the upper barrel socket via live fire testing to get the rear sight dead center for zero.

As for the problem with the front sight, a New .040 longer sight post should resolve the problem there. Bushmaster carries the longer posts for under $10.


We opened the right side of the notch maybe 1/32 to 1/16 of an inch, I'd guess.

The degree that this rifle was off was quite a bit - we were 28 clicks left on windage, which pretty much used up all of the windage adjustment on the rear sight. A small adjustment was not called for here.

I have another rifle that's zeroed at 12 clicks left, and I may try the "mallet method" on it, just for an experiment.

Regarding this "mallet method" - Where is the actual movement coming from? When we took our rifle apart, the pin on the barrel extension fit into the notch in the socket on the front of the receiver with little to no play. As far as I can guess, there are three places where there is "potential give" in this assembly - fSB to barrel, barrel to barrel extension, and barrel extension to receiver. Which one "gives" (or is it all three)?
Link Posted: 12/16/2008 1:48:35 PM EST
Originally Posted By Dano523:
Originally Posted By kilo_juliet:
Originally Posted By tpelle:


filed the barrel locating notch in the receiver an appropriate amount, then re-assembled it. It now shoots pretty much to POA.




Will this change POA for the next barrel?

How much did you need to file off?


Since sending the upper back appears to not be the option, lets just deal with the problem instead.

Take the upper off the lower.

Take a look at the direction that the rear sight has to be moved back to center, this is the direction that the barrel needs to be slipped in the upper barrel socket.

Now lay the upper in you lap with the receiver on one leg, and the barrel just behind the FSB across your other leg.

Using a leather mallet (will not mar the finish) and holding onto the receiver only, make sharp blows to the FSB just above the barrel to slip the entire barrel in the upper barrel socket. The FSB is taper pinned to the barrel, so it will not rotate, but since the barrel is retained in the upper barrel socket via tension alone, the barrel is going to slip in the receiver.

If you do this at the range, then you can slip the entire barrel in the upper barrel socket via live fire testing to get the rear sight dead center for zero.

As for the problem with the front sight, a New .040 longer sight post should resolve the problem there. Bushmaster carries the longer posts for under $10.





Is there really that much play between the barrel pin and receiver notch ? i would think there is not enough play to adjust his windage problem. It seems to me this method would just be forcing the pin against the notch and either bending the pin or enlarging the receiver notch. I would think that most windage problems like this are caused by a canted front site post or the exstension not aligned with the front site post when installed . Also when enlarging the receiver notch and turning the barrel you are also moving the exstension lugs and how they align with the bolt lugs.
Link Posted: 12/16/2008 1:51:54 PM EST
[


Regarding this "mallet method" - Where is the actual movement coming from? When we took our rifle apart, the pin on the barrel extension fit into the notch in the socket on the front of the receiver with little to no play. As far as I can guess, there are three places where there is "potential give" in this assembly - fSB to barrel, barrel to barrel extension, and barrel extension to receiver. Which one "gives" (or is it all three)?




You would think it would be the receiver notch since it aluminum.
Link Posted: 12/16/2008 2:25:41 PM EST
Don't modify upper or barrels to "fix" canted front sight bases without paying attention to the alignment of the barrel extension. If the barrel extension (locking lugs) are rotated in the upper there will be a significant potential for excess wear, functional issue as well as potentially safety issues.

Excess left windage is common for uppers where the barrel wasn't properly installed, but look over the situation before committing to a course of action. If the FSB is canted from the TDC of the barrel as determined by the barrel extension you're screwed, send it back to the manufacturer.
Link Posted: 12/16/2008 2:44:51 PM EST
Originally Posted By Gamma762:
Don't modify upper or barrels to "fix" canted front sight bases without paying attention to the alignment of the barrel extension. If the barrel extension (locking lugs) are rotated in the upper there will be a significant potential for excess wear, functional issue as well as potentially safety issues.

Excess left windage is common for uppers where the barrel wasn't properly installed, but look over the situation before committing to a course of action. If the FSB is canted from the TDC of the barrel as determined by the barrel extension you're screwed, send it back to the manufacturer.


Good point.

What assurances are there that the mfgr isn't just filing the upper? Is there some type of gauge they use to measure extension rotation? What's the spec on just how much rotation the extension can take before locking becomes compromised?
Link Posted: 12/16/2008 7:24:12 PM EST
Link Posted: 12/16/2008 7:54:14 PM EST
[Last Edit: 12/16/2008 7:55:54 PM EST by Keith_J]
Take it apart. Regrease the threads with moly only grease. Hand tighten the nut. Then boresight with the rear sight CENTERED. Adjust windage by torque on the barrel. Once it is zeroed on windage, clamp the upper in the block firmly in a good vise. Now with a second vise, like a drill press vise, clamp the barrel to the bench. Now the barrel AND the upper receiver are locked into position. Torque the nut and check the boresight, it should be on the money.

I get within an inch at 100 yards with this method. Precise and easy.

Why? The torque of the nut WILL torque the notched part of the upper. With proper grease and locking the upper to the lower, relative slip between the parts assures it is the threads, not the shoulder of the receiver extension.
Link Posted: 12/20/2008 10:35:09 AM EST
I inspected the barrel and receiver assembly. Rebuilt it all up. Nothing seemed wrong. With the witness marks on the rear receiver lined up in the center and on a distant plub line with the boresight, all seems AOK.

Yet the point of impact is still 4" to the right at 25 yards.

This is crazy. I checked to see if it was me with an older AR. Nope, I still know how to shoot these things. It is alost like the muzzle break or the crown is doing something.
Link Posted: 12/20/2008 12:57:55 PM EST
[Last Edit: 12/20/2008 12:58:16 PM EST by j3_]
I got a flat top a few months back that was a problem. The rail was not machined true to the upper and ran off on a slight angle. Caused a sighting problem
Also putting an upper on with upper receiver blocks instead of barrel blocks can cause it to be off where the rear sight has to be run to the left.
Link Posted: 12/20/2008 3:59:20 PM EST
I once used one of those cheap construction lasers to check for a canted sight on an AK47, i first put the receiver in a vice and made sure it was level, then sat the laser on the receiver and shot a laser line down the barrel and front sight. If you dim the lights in the work area the laser line shows up real good. I was surprised how well it worked.
Link Posted: 12/26/2008 5:43:20 AM EST
I have had trouble with this in the past but not anymore.

The first thing I do on a build is to check the upper threads to make sure that the barrel
will have a flat surface to mount in.

Place the upper on a piece of glass , barrel threads down and use a good square up against the top edge of the receiver then look to see if the threads lay flat on the glass(I have seen more that dont than do).
get the upper reciever lapping tool form Brownells( not expensive) use a little alumninum oxide paste(vavle grinding paste) on the front of the threads and run the lapping tool with the drill for 10 seconds , it will flush up the upper so that when the barrel is installed it will be inline with the upper reciever axis.



There is a thread someplace on here with pics of how to do this.
Link Posted: 12/26/2008 6:12:50 AM EST
[Last Edit: 12/26/2008 6:13:37 AM EST by j3_]
Originally Posted By Jusbo47:
I have had trouble with this in the past but not anymore.

The first thing I do on a build is to check the upper threads to make sure that the barrel
will have a flat surface to mount in.

Place the upper on a piece of glass , barrel threads down and use a good square up against the top edge of the receiver then look to see if the threads lay flat on the glass(I have seen more that dont than do).
get the upper reciever lapping tool form Brownells( not expensive) use a little alumninum oxide paste(vavle grinding paste) on the front of the threads and run the lapping tool with the drill for 10 seconds , it will flush up the upper so that when the barrel is installed it will be inline with the upper reciever axis.



There is a thread someplace on here with pics of how to do this.



I would check the inside of the upper with the barrel seated first before removing any finish or material from the face of the upper receiver. Some barrel extensions seat flush with the inside edge of the upper and removing material from the face of the upper can cause the extension to seat further leaving a lip inside of the upper for a bullet tip to hang when feeding in.
Link Posted: 12/26/2008 6:54:56 PM EST
Last resort is to put the upper in the block and hold in vice, put a steel rod through the front sight base and twist the front sight in the direction it needs to go. (This is the same concept as the previous post where you "whack" the front sight with a hammer) This won't work if you have the barrel really tight on the receiver, but if there is enough room in the upper receiver "slot" and the barrel nut is not too tight, the whole barrel should move with enough pressure on the rod. We use a 2' long 1/2" steel rod, but a breaker bar or real long heavy duty screwdriver would work as well. This is very common practice and won't hurt anything if you use moderation. If your barrel alignment pin is up against the slot in the upper, then this will not help, you will need to send it back. It is possible that the gas port and the alignment pin are out-of-line.

You need to slide the breaker bar all the way through the front sight base "triangle", so that you get an equal length on both sides and can grab the bar and twist with equal force on both sides of the barrel/fsb. This way you get equal torque and do not bend or push the front sight, but get an actual "twisting" force on the barrel.
Link Posted: 12/26/2008 7:39:43 PM EST
I too have seen this happen far too often. I don't think I would trash an upper to fix the canting. The problem seems to be the barrel index pin or FSB was not put on at exactly TDC on the barrel. I have seen Colt, Bushmaster, and Stag barrels that have a slight cant to the left or right. I have pulled them off and the barrel pin on top of the extention is not bent, there is no play in the receiver notch. I figure the FSB is not perfectly at TDC. Seems like this should not be happening too much. Perhaps the mgf need to take a little more time to plum the FSB to the index pin.
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