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Posted: 11/28/2009 11:38:17 AM EDT
| I have an ACOG that I use on three different guns all of which are different brands (RRA, Armalite, and BCM). My problem is that I can switch the ACOG back and fourth between the RRA and Armalite with very little adjustments neccessary to re-zero however, when I put the ACOG on the BCM, the groups move to the left by about 16 inches at 100 yards. This seems to be alot to me. Is this acceptable? If not, what can I do to fix it? |
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Short of sending the BM upper back to see where the problem lies (either bore line not on the same plain as the upper receiver line, or the flat top rail being off as well), you will need to rezero out the scope for the rig that is will be dawned on.
But, knowing what the each rifle zero is needed for that rig, would allow you to swap and quickly adjust the scope as needed. |
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Short of sending the BM upper back to see where the problem lies (either bore line not on the same plain as the upper receiver line, or the flat top rail being off as well), you will need to rezero out the scope for the rig that is will be dawned on.
But, knowing what the each rifle zero is needed for that rig, would allow you to swap and quickly adjust the scope as needed. |
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You should not be switching optics between uppers. The optic is designed to be zero'd to one upper and left in place. Generally, Loctite is used to make sure it never moves. Swapping it around violates the entire concept of it holding zero.
There is nothing wrong with the upper for shooting to a different POI than the other uppers...it is a different upper. Put an optic on each one and use witness marks to lock everything down, then leave it alone. |
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Lapping an AR upper is not the same as lapping a bolt action. It does not affect head space. The AR's HS when the extension is installed. All lapping does is align or true the upper face to the extension.
Or you could just smack the FSB in the desired direction or file out the pin's groove to allow rotation and shim it when you go too far. Never seen a stripped upper that had any evidence of that kind of adjustment. 458 |
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Quoted:
You should not be switching optics between uppers. The optic is designed to be zero'd to one upper and left in place. Generally, Loctite is used to make sure it never moves. Swapping it around violates the entire concept of it holding zero. There is nothing wrong with the upper for shooting to a different POI than the other uppers...it is a different upper. Put an optic on each one and use witness marks to lock everything down, then leave it alone. There's nothing wrong with changing optics from one rifle to another. Just be aware that a change in zero is to be expected. I have a couple of rifles with spare scopes and each will retain zero with it's intended scopes. However if I move a 1.5x from a .375 to .458 it will require a re-zero. A good scope and mount will allow you to just return to the original setting and be back very near zero. No glue involved. This may seem like an odd concept to some. Anyone who has learned how to use a well made scope will realize those external W/E knobs can be used. As well as the Mil-Dot reticule. 458 |
| Thanks for the advice guys. I'd love to buy an eotech xps so I won't have to switch optics arount but I've been hearing alot of bad stuff about them. Plus it seems they are really trying get rid of their products right now by offering that rebate. This means that they are either trying to avoid paying taxes for a bunch of overstock or they are going to be making some changes for the first of the year. What do yuins think? Sorry for drifting off topic |
| I realize the zero will not be the same on all riflels. I am concerned with the amount of adjustments that need to be made. Very little elevation changes are needed but almost a foot of windage at 50 yards cannot be right. It seems to me that the barrel is not seated straight in the receiver. When zeroing in with iron sights I experience the same thing. I have to adjust my rear sight far to the right to get a good zero. I do not have these problems with my other two carbines. |
| Thanks. BCM return policy is only 30 days and I bought this last spring. I just now started shooting this upper though. I will probablly buy the stuff to lapp the receiver since I have a noveske barrel that I plan to mount to another receiver one of these days. Anyone have a link for propper procedure for lapping including what grit compound to use? |
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Quoted:
Thanks. BCM return policy is only 30 days and I bought this last spring. I just now started shooting this upper though. I will probablly buy the stuff to lapp the receiver since I have a noveske barrel that I plan to mount to another receiver one of these days. Anyone have a link for propper procedure for lapping including what grit compound to use? Well, it is BCM after all, try giving them a ring. Never hurts to ask. 30 days is to cover their ass legally. |
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Check barrel straightness, if the barrel is bent the other tests won’t be right.
A gage is best, you might try a flexible rod that is near bore size. Edit to add: The goal is to show if the muzzle and the breach end of the barrel are not pointed exactly 180 degrees apart. As bad as your barrel sounds you could probably see it if the barrel is bent. I said flexible rod because we have some delrin rods laying around at work and if you pushed a straight rod through the bore with the ends of the rod sticking out it should be easy to see if the rod is still straight. Another possibility find a drill bit the right diameter and cut the flutes off to make a drop through gage. If the barrel is straight take a long cleaning rod put some o-rings on it to keep it centered in the bore and run it through the bore. If the rod is centered in the rear of the upper then check the sights. Looking through the barrel bore at a vertical line in the distance with the upper not canted see if the sights line up on the same line. The TM has a better description of this. There should be something obvious like the FSB canted. If the cleaning rod is not centered then pull the barrel. Look for any foreign objects that might be preventing the barrel from seating solidly and squarely on the upper, same for barrel nut. Look for any gross defects in the upper and the barrel. Is the barrel extension screwed straight on the barrel? Use the cleaning rod trick to see if the bore is centered in the barrel. If the face of the upper is not square to the centerline of the upper as someone else posted you will have to lap it. |
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