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Posted: 8/19/2004 5:12:56 AM EST
[Last Edit: 8/19/2004 5:19:15 AM EST by Screech]
I just assembled a 20" rifle. All parts except barrel are RRA. The barrel is a Bushmaster Gov. profile 20". This gun shoots to the right 3 inches @25 yards with the rear sight all the way left. What can be done to correct this? Thank You.

Edited to add: FSB looks straight. When I assembled this rifle the alignment pin was a super tight fit in upper receiver. Also only took 40 foot pounds or so to align gas tube. Thanks
Link Posted: 8/19/2004 7:30:49 AM EST
[Last Edit: 8/19/2004 7:33:02 AM EST by Dano523]

Originally Posted By Screech:
This gun shoots to the right 3 inches @25 yards with the rear sight all the way left. What can be done to correct this? .

Edited to add: FSB looks straight.



The front sight is canted to the left. When you install a barrel, you need to bore sight, and confirm that the sights are aiming at the same point down range as the bore.

Now, if your groups are shotgun patterning, then you may have bullet strikes on the FS/Brake.

Bottom line is if the barrel is holding a grouping (no matter where the group is in regards to the where the sights are looking), then you need to pull the barrel, re-index the front sight/barrel where it needs to be to zero the sights, and re-tighten down the barrel nut. My guess is you forgot to bore sight to confirm that the sights are correctly indexed with the bore line after you installed the barrel.

Once you get the front sight bore sighted (barrel indexed correctly), you can use a leather mallet to fine-tune the front sight (drifting the entire barrel in the upper barrel socket) under liver fire conditions (barrel harmonics) to fully center out the rear sight to have a dead center zero (repeatable return mark for still wind conditions).
Link Posted: 8/19/2004 9:42:22 AM EST
Thanks for your help. My groups are under 2 inches @ 35 yards w/gun resting on my trucks door. I think I need to remove the barrel and start over.

What do you mean by "bore sight"? Do I need a to invest in a bore sight instrament?


Once you get the front sight bore sighted (barrel indexed correctly), you can use a leather mallet to fine-tune the front sight (drifting the entire barrel in the upper barrel socket) under liver fire conditions (barrel harmonics) to fully center out the rear sight to have a dead center zero (repeatable return mark for still wind conditions).



Can this be done with the rifle fully assembled?

Again, Thanks for your help.
Link Posted: 8/19/2004 12:00:41 PM EST
With the upper still in an upper vise block (such as the open top design) point the barrel bore at a distance target, then look down the sights. Both should be looking at the same point (vertical).


In regards to drifting the barrel in the upper socket for fine-tuning with the rifle completely built, you hold only the upper receiver, and use a leather mallet to make a blow at the base of the sight tower to drift the entire barrel in the upper barrel socket. Unlike a threaded barrel into a receiver, the AR barrel/extension is tension retained by the barrel nut only. The barrel extension pin is used for indexing the barrel, but due to methods of barrel installation, the barrel can rotate due to either the pin slightly bending, slop in the upper socket slot, or the pin digging into the upper pin socket when the barrel nut is tightened. This method of drifting the entire barrel using a leather mallet is very, very faster than pulling the barrel and shimming the pin into a new desired location in the socket slot.


_______________
Some will post saying that shimming the pin is the better way, and at one time I used to correct the final firing for effect zero that way (rear sight centered). This is when I had way too much time on my hands and could screw around resetting the barrel index until I get it just were I want to be in regards to the front tower indexed to center out the rear sight for zero (read hours including fire testing). Now with the leather mallet, the final fine tuning of the index of the front sight tower under live fire takes me around 10 shots, at the range, and is completed in less than a few minutes.

As for the barrel holding it zero by either leather mallet, or removal and shimming, they will both hold index the same. If the barrel takes a hit hard enough to drift the front tower if leather mallet drifted, then even it the extension pin is shimmed tight, it will still either bend the pin or the pin will indent the slot causing a front tower index shift.
Link Posted: 8/20/2004 3:29:16 AM EST
Dano523, Thank You for taking the time to help me out. I feel confident now I can get this problem fixed, Again Thanks!
Link Posted: 8/20/2004 4:08:03 AM EST
Dano, are you directing the blow to the barrel sideways, or trying to "rotate" the fsb in the desired direction?

Paladin
Link Posted: 8/20/2004 7:44:57 AM EST
[Last Edit: 8/20/2004 7:46:13 AM EST by Dano523]
The leather mallet strike is at the base of the front sight tower just above the barrel, and in the direction that you want to rotate the entire barrel in the upper socket. This prevents the tower from being bent or broken as if you were to make contact at the top ear section (a cast FSB may still crack). Again, this rotates the entire barrel in the upper barrel socket, and not the front sight tower on the barrel.

On a barrel that the FSB and the barrel extension pin are not correctly indexed (not on the same index plain), then the only feasible option is to go to a set screw application instead of the two taper pins. 99% of the time when the two are not indexed correctly, the gas port is still in line with the receiver extension pin, and just the FSB was not jigged correctly when it was installed/drill for the taper pins.
Link Posted: 8/20/2004 7:59:57 AM EST
Dano,
I, too have this problem. I have been slugging the FSB while holding the
complete rifle on a stool. At one point I sat on the rifle so that
it was completely supported. I began hitting the FSB with a dead blow hammer.

I struck hard nothing moved. Do I need to use a sledge hammer ?

Mike T
Link Posted: 8/22/2004 11:24:10 AM EST
Mike,
I think it may be time to pull the barrel and check the FSB to the receiver extension pin to confirm that they are on the same plain. Drifting the barrel can be used to bring the FSB over a few clicks, but if the rear sight is cranked all the way over, then it's a index problem with the pin/FSB that will need to be resolved first.

Again, the leather mallet is only used to fine tune, and not a shortcut for the initial bore sighting during install, or to correct a incorrectly installed FSB.
Link Posted: 8/23/2004 1:53:24 AM EST
Thanks.
Will pull apart, and see what is wrong.

Mike
Link Posted: 8/26/2004 11:04:02 AM EST
Sounds like the infamous Bushmaster misaligned front sight base/barel index pin. I have a Dissy upper that was real bad like this. It shot way left even with the rear sight adjusted to max windage. I pulled the barrel, tried shimming, tried the hammer trick, but never could get it right. I had to pull the front sight base and drilled and tapped two set screws into the bottom of the front sight base so I could adjust the front sight base by turning it to a zero and then tightening the screws. Once sighted in, I redrilled the pin slots and locktited everything down with the green wicking locktite.

Of course the Dissy front sight base has no relation to the gas tube or gas port, so this was no problem. Our original poster probably can't do this. If you can't fix it by the ways described above, I'd contact Bushmaster for correction or replacement.
Link Posted: 8/27/2004 12:00:27 AM EST
[Last Edit: 8/27/2004 12:01:49 AM EST by Dano523]

Originally Posted By GLC1:
Sounds like the infamous Bushmaster misaligned front sight base/barel index pin.



Thats a new one to me,

I'm only aware that the someone over there needs to either learn to read a torque wrench and correctly stake the allen key screws, or they need to find a new supply of screws since they are having a big problems with their keys screws becoming loose and causing short stroking due to leakage.
Link Posted: 8/27/2004 12:37:33 AM EST
Link Posted: 8/27/2004 3:06:36 PM EST

Originally Posted By Tweak:

Originally Posted By Dano523:
Thats a new one to me,



They used to have quite a reputation for rear sights saturated to one side. Was usually mis installed barrels not an improperly installed FSB.



I had one like that also ....they fixed it without a problem, but I had to wait 6 weeks to get it back!
Link Posted: 8/28/2004 1:15:02 AM EST
This Bushmaster problem was really bad probably about 6-7 years ago when I bought this Dissy barrel. They seem to have been much better since then with less problems like this reported in the forums.


Back then, I remember many forum threads about all the alignment problems and the long wait to get it fixed if you sent it back. I didn't feel like waiting 6 weeks so I fixed it in about an hour by myself.


Works fine now, zero is only three clicks off the center mark.



Link Posted: 10/4/2004 10:00:15 AM EST
tagged.... I may have this problem :(
Link Posted: 10/4/2004 10:08:28 AM EST
To fix my rifle I ended up taking it apart and filing the slot in the receiver so the barrel could be twited slightly. When I re-assembled the rifle I had a buddy keep upright pressure on the barrel. To do this we used a big wooden clamp on the front sight base. The rifle now shoots center of target with rear sight in the center. Good Luck
Link Posted: 10/5/2004 3:40:48 PM EST
That was the best way to do it. I have had to file the index slot on a couple of rifles too. If you have a bbl. with a pinned fsb that is all you can do. Now I will only buy bbls. without fsb's and install fsb's that come with the setscrews in the bottom or sides, it make assembly and zeroing much easier.
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