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Posted: 11/5/2011 5:51:54 PM EDT
| I recently changed out the upper on my Bushmaster AR (with an A1 upper originally) for a flattop that I got at Palmetto State Armory. I used all the original parts fom the Bushmaster upper including the barrel. I also mounted a Magpul MBUS rear sight. Before the change it was very accurate without any sight changes. Now it is shooting VERY FAR right and SUPER high. I was able to get it sighted in after more than 60 rounds but my rear sight is almost all the way left and my front sight post is almost all the way out!! This can't be normal. Should I pull the barrel and start over? Any other suggestions? Any help is much appreciated. Thank you. |
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thats normal
Most magpul rear sights I have seen (Including mine) are way left And normally on a flattop upper your supposed to have an F-Marked FSB, which is slightly taller Bushmaster sells a tall front sight post that is supposed to be used with a flat top and normal FSB like yours (Because they dont use F marked FSB on their flattops for some reason) So if you want, just get the tall post from bushmaster and your good |
| Thanks guys. That makes me feel a little better. I'll install a taller FSP since I won't be able to get a new FSB past the boss (wife) right now. I still don't understand the windage though. I have a buddy that installed one on his AR and it was dead on right out of the box. Maybe he just got lucky. I'm going to call Magpul next week and ask them about it. I'll post their reply when I get one. If anybody else has any suggestions regarding the windage issue I'm still all ears. Thanks again. |
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That sounds like way too much windage.
Maybe the front sight post being almost out is a factor. I am guessing that if the Magpul sight is the problem they will replace it. I would personally want to know for sure the windage was not caused by the upper’s fit to the barrel. YMMV |
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Not normal! That is way too much windage even for a Magpul MBUS. If it was shooting "balls on" prior, the upper receiver is probably out of square. Sounds like it is having an issue with a bit of flashing, or it's out of square. When you tighten the barrel nut, it pulls the barrel in God aweful directions sometimes.
I think I know of something that can help you - The upper can be lapped. Video Take the upper apart, and buy one of these...Lapping tool. It can be done on a drill press. Use some grinding compund that is good for Aluminum - Like Aluminum Oxide...Link. Please do more than the 600 and 800 that come in the twin pack that Brownell's sells...Takes all day. Try and get some 150 or 220, and then get some 600 or 800 and polish that sucker. You will notice a HELL of an improvement right off the bat. How about some pictures of the RIG? It could be a canted FSB. Do you have Magpul MBUS front and rear? I still have never had one that far out unless I didn't true the receiver. |
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Quoted:
Not normal! That is way too much windage even for a Magpul MBUS. If it was shooting "balls on" prior, the upper receiver is probably out of square. Sounds like it is having an issue with a bit of flashing, or it's out of square. When you tighten the barrel nut, it pulls the barrel in God aweful directions sometimes. I think I know of something that can help you - The upper can be lapped. Video Take the upper apart, and buy one of these...Lapping tool. It can be done on a drill press. Use some grinding compund that is good for Aluminum - Like Aluminum Oxide...Link. Please do more than the 600 and 800 that come in the twin pack that Brownell's sells...Takes all day. Try and get some 150 or 220, and then get some 600 or 800 and polish that sucker. You will notice a HELL of an improvement right off the bat. How about some pictures of the RIG? It could be a canted FSB. Do you have Magpul MBUS front and rear? I still have never had one that far out unless I didn't true the receiver. Agreed. Sounds out of square to me. Get a piece of white thread or light string, tie a fat knot in it and thread it through your rear peep. Pull tight to the front post. See if it runs true down the center of the rifle. Helps if the peep is centered.. |
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Quoted:
Not normal! That is way too much windage even for a Magpul MBUS. If it was shooting "balls on" prior, the upper receiver is probably out of square. Sounds like it is having an issue with a bit of flashing, or it's out of square. When you tighten the barrel nut, it pulls the barrel in God aweful directions sometimes. I think I know of something that can help you - The upper can be lapped. Video Take the upper apart, and buy one of these...Lapping tool. It can be done on a drill press. Use some grinding compund that is good for Aluminum - Like Aluminum Oxide...Link. Please do more than the 600 and 800 that come in the twin pack that Brownell's sells...Takes all day. Try and get some 150 or 220, and then get some 600 or 800 and polish that sucker. You will notice a HELL of an improvement right off the bat. How about some pictures of the RIG? It could be a canted FSB. Do you have Magpul MBUS front and rear? I still have never had one that far out unless I didn't true the receiver. Thank you. I was thinking this might be a possible culprit. I kept the standard FSB but that shouldnt pose much of an issue. My buddy uses the standard FSB and his shoots dead nuts on. That was one of the things that sold me on the MBUS. I'll try to take and post some pics. I didn't notice the FSB being canted but I haven't checked it yet to be sure. I'll check it this week to see. I'll also try to get a lapping tool and see if that helps. I'm still trying to get ahold of a person at Magpul. |
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