Question

[rwsorensen]

I built an M4 with a DPMS 16" heavy contour barrel. I am a Combat Arms Instructor in the AF, re barreling M4/M16's is something I do on a regular basis. The problem I have is that I cannot get it to zero with a BUIS. It shoots about a 12" to the right and 12" high, to move the front sight post down that far would mean taking it all the way out. anybody had this problem before? what would be the issue? It is a brand new barrel and it is only torqued to 42 FT LBS. Would switching the front sight post with a shorter one work? The rear sight is also an issue, with it pegged to the far left it still shoots 7" to the right. Thoughts?

[MistWolf]

It shoots high & right at what range?

A shorter front site post will only make the problem worse. You move the front sight the opposite direction you want to move the point of impact.

Your FSB may be canted, or the barrel bent. This will cause your windage to be off. I've also heard stories that barrels can be cocked to one side if not properly installed.

Other possibilities lay in the BUIS themselves and how they mount to the rails. If the front sight is removable and not an FSB, the rail it mounts to could be cocked in relationship to the rail of the upper. Again, it could also be a problem with the detachable front sight itself

[Hmac]

What kind of Front sight are you using? If it's an FSB, sounds like it might be canted when you reinstalled it. If it's rail mounted, maybe the rail was misaligned on the reinstall.

[Eric D.]

If you're doing zeroing at 25 yards you shouldn't have trouble setting elevation. Since its way high I'd assume you're zeroing at a further distance in which case you should make sure you have the right height FSB.

Unless the FSB itself is bent, it cant technically be canted like AK sights can be. The indexing pin at the barrel extension determines the radial orientation of the FSB and the barrel needs re-indexed.

You're right, swapping a barrel is pretty straight forward and it's kinda hard to screw up. I would question the integrity of the barrel itself (bent, improperly indexed etc.) Try to get in touch with DPMS and see if they can help you out.

[Iraqgunz]

If the holes were drilled improperly and the FSB was then pinned it could in fact be canted.

I would remove the barrel and check to make sure that it is properly seated and then tighten it back down.

Since you didn't mention the distance that would be nice for us to know. Also maost of their barrels are 1/9 so depending on what ammo you are using that could also be an issue.


If you're doing zeroing at 25 yards you shouldn't have trouble setting elevation. Since its way high I'd assume you're zeroing at a further distance in which case you should make sure you have the right height FSB.

Unless the FSB itself is bent, it cant technically be canted like AK sights can be. The indexing pin at the barrel extension determines the radial orientation of the FSB and the barrel needs re-indexed.

You're right, swapping a barrel is pretty straight forward and it's kinda hard to screw up. I would question the integrity of the barrel itself (bent, improperly indexed etc.) Try to get in touch with DPMS and see if they can help you out.

[Eric D.]

If the holes were drilled improperly and the FSB was then pinned it could in fact be canted.

I don't mean to butt heads with you. I say it can't be canted becasue the FSB's realtionship to the barrel is only critical where the gas holes line up. The pins determine the FSB relationship to the barrel and the indexing pin determines the barrel's realtionship to the receiver. So by transitivity the FSB position is a function of the barrel's relationship to the receiver. My argument is that, as long as the gas holes line up, the pinning of the FSB is irrelevant. Its final position will be determined by the indexing pin. I guess its coming down to viewpoint: You say the FSB is canted, I say the barrel is canted.

[rwsorensen]

It's not a removeable front sight, it has an A frame. I am zeroing at 25m. The barrel is not bent or canted, it has a free float rail system on it. Any other ideas? I have used 55, 60, 62 grain ammo to rule that out. i have already taken the barrel off twice to make sure it's seated properly and not torqued too much. This isn't my first rodeo, out of hundreds of barrel changes this is the first one I've had a problem with.

[Eric D.]

It's not a removeable front sight, it has an A frame. I am zeroing at 25m. The barrel is not bent or canted, it has a free float rail system on it. Any other ideas? I have used 55, 60, 62 grain ammo to rule that out. i have already taken the barrel off twice to make sure it's seated properly and not torqued too much. This isn't my first rodeo, out of hundreds of barrel changes this is the first one I've had a problem with.

Try putting a laser bore sight in it and seeting it up in a stand. Look at it from a bunch of different angles and see if anything jumps out at you. Your ammo should be fine with a 1/9 twist. If it were me I would have the barrel inspected for straightness, concentricity, etc. Perhaps the face where the barrel butts up against the upper is not square with the axis of the receiver or the hole the barrel slides into is not colinear with the the receiver?

[rwsorensen]

I ran a straightness gauge through it, so it's good there. I am thinking about trying a different upper, if that's the issue, i'll fix it. Thanks for the advice.