AR Sponsor
Posted: 11/11/2009 6:06:21 PM EDT
|
I took a new Bushmaster M4-gery to the range for the first time. At 50 yards it wouldn't even hit the paper, I discovered the rounds were hitting far left of the point of aim. After adjusting the windage nearly all the way to the right I was able to zero it. Is a canted front sight base the most likely cause of this? Any tips to measure if it is dead center on the barrel? Could the barrel be over torqued causing this? Are there any other likely causes I need to check?
I have two Colts that were drop dead center right out of the box, I'm really disappointed with this Bushmaster. |
This is a common problem and has been addressed a lot. Have yet to read how any addressed the out come.
If you assembled it yourself. Pull it apart and buy a receiver lapping tool from Brownells or Midway with compounds (Aluminum Oxide) $30-50. Lap re-assemble and try again. If you got an assembled upper, send it back. 458 |
| I had a Stag like that. I couldn't tell that the front sight was canted by looking, but if you put it in a vise and leveled the gun (with a bubble level), then the wings on the front sight wouldn't be level when you put the level across them. In my case, I had a gunsmith replace it with a fold-down front sight anyway. |
|
Quoted: This is a common problem and has been addressed a lot. Have yet to read how any addressed the out come. If you assembled it yourself. Pull it apart and buy a receiver lapping tool from Brownells or Midway with compounds (Aluminum Oxide) $30-50. Lap re-assemble and try again. If you got an assembled upper, send it back. 458 http://www.brownells.com/.aspx/pid=20220/Product/AR_15_M16_UPPER_RECEIVER_LAPPING_TOOL Quoted: I just turn it upside-down on a true/flat tabletop and look to see if all 4 ears of the front/rear sights are all touching at the same time. Sometimes loosening the barrel nut will allow you to rotate a * or two. I saw a guy at a range whack his FSB with a ball peen hammer one time (I saw it one day, he did it several times while I watched) to 'true it back up'. If I recall correctly, it had something to do with dropping the weapon, but i'm not clear how far it was dropped, or how many times. I had a Stag like that. I couldn't tell that the front sight was canted by looking, but if you put it in a vise and leveled the gun (with a bubble level), then the wings on the front sight wouldn't be level when you put the level across them. In my case, I had a gunsmith replace it with a fold-down front sight anyway.  |
|
All fours "ears" may not be present.
People in VA are weird. But at least they know how to execute an murderer.  Good job
You would be assuming all points and surfaces were true with the up side down method. The FSB and reciever are forged parts that could screw up this. Good idea for a flat top though. But still at best, a good guess 458 |
|
I don't thing there is a real fix to this problem as it occures when the FSB is drilled an installed on the barrel. Most people including me can't see the degree of "cant" in the FSB until the barrel is installed on a upper receiver. I have had several barrels from, BM, Colt,, LMT, Sabre, Stag all with canted front sight bases, most to the left, a couple to the right.
I have a new 20" Colt barrel that is has this issue. I put it in 5 different uppers and it's canted in the same degree to the left pre and post install so tightening the barrel nut is not the cause. Also, I have looked at the receiver notch and barrel index pins and there is no deformation, the fit is still tight. The only evidence of a overtorqued barrel nut I could imagin is if the index pin and notch were damaged to the point there was 2-3 degrees of play, and that would take a lot of torque. I think the problem rests with the mfg, installing the FSB off TDC. |
| Very common problem. I recently adjusted an old J&T upper by loosening the barrel nut a bit first, and then there was enough clearance in the indexing notch to straighten it a bit. Still not perfect, but at least it will zero without cranking all the way to the left. |
AR Sponsor