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Posted: 9/18/2011 6:43:11 PM EDT
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I have installed 4 of these forearms on builds before without issues and have in fact been a huge advocate of them. However on my latest build, I ran into an issue. I'm using a 13" rail and a DD lo pro gas block. I have installed and reinstalled this forearm four times, using a LT scope mount to bridge the gap between the forearm and the receiver as I always have, yet the forearm continues angle upward so much to where the forearm comes within less than a millimeter to hitting the gas block. In fact, letting the bolt slam home on an empty chamber I can hear a clang of the forearm hitting the gas block. My brother has an identical build with an identical barrel (both custom made wildcats using the same reamer and everything) and my rifle shoots horribly compared to his, as in 3-5 MOA compared to .75-1.5. His rifle has the same forearm and gas block too and there is plenty of clearance around his gas block. I attribute this difference in accuracy to the gas block impacting the forearm (this is a custom very thin profile so I suspect any pressure on the barrel will greatly influence accuracy).
My guess is that the threads on the upper receiver are cut at a cant, causing the barrel nut to be canted and therefore the forearm. Does this sound reasonable? Does anybody have any tips, suggestions, anything that can recommended? If I can't salvage this rig, I'm going to skip this forearm and rebuild it with a monolithic upper. |
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Put a straight edge on the rail and see if its flat. in order I would guess bad upper bad forearm bent barrel. I just did. The straight edge rests on the upper, then on the forward end of the forearm. There is a gap between the straight edge and the forearm from the receiver interface and about 10" of the forearm. So the forearm definitely is curving upward. Do you think that the gas block hitting the forearm would result in such bad accuracy? |
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Pull it all apart. Use a machinists square to see if the face of the upper is not true to the picatinny rail. You may have just a dud upper receiver.
My 16" Grendel exhibited similar characteristics until I lapped it with the Brownells AR15 lapping tool and 180 grit abrasive compound. The 16" barrel was pointed left, almost to where the barrel looked in line with the left edge of the picatinny rail at 11 inches. This was using an Alexander Arms upper which had previously been barreled as a 20" weapon. After running the lapping tool and getting the face where it should've been it corrected it. Make sure there aren't any pieces of dirt or anything in the receiver or against the barrel extension and clean out the barrel nut also. You may have to switch to another upper and I'd be willing to bet it corrects the problem. |
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Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. A badly installed barrel extension can do the same, though unlikely. 3-5MOA means something is very wrong if you are using good ammo... but it could be a bad barrel there as well, since you arent using a known good barrel. I never recommend using an optic mount and bridging the gap between the upper receiver and a handguard. That is always a bad idea. |
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Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. A badly installed barrel extension can do the same, though unlikely. 3-5MOA means something is very wrong if you are using good ammo... but it could be a bad barrel there as well, since you arent using a known good barrel. I never recommend using an optic mount and bridging the gap between the upper receiver and a handguard. That is always a bad idea. Why is bridging bad for installation? I don't shoot it this way, but when I install rails that have a same height 12 o clock rail as the receiver, using a LaRue mount across both ensures that the rails are lined up. I have always had success doing it this way in the past. |
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Quoted: Quoted: Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. A badly installed barrel extension can do the same, though unlikely. 3-5MOA means something is very wrong if you are using good ammo... but it could be a bad barrel there as well, since you arent using a known good barrel. I never recommend using an optic mount and bridging the gap between the upper receiver and a handguard. That is always a bad idea. Why is bridging bad for installation? I don't shoot it this way, but when I install rails that have a same height 12 o clock rail as the receiver, using a LaRue mount across both ensures that the rails are lined up. I have always had success doing it this way in the past. 1. Rails can and will move, at the worst times. Even if it is ever so slight. This will throw off your zero. 2. Look at a gun firing in slo-mo - there is a LOT of flex that occurs and this is bad for the mount, and eventually the zero. |
| i've got a troy rail. when i asembled it i had the same angle up thing going on. i loosened up the pinch bolts and flipped the upper and rail over. set them on a flat surface and with a little pressure on the bottom of the rail and upper retightened the pinch bolts. was straight after that. had a tiny gap between upper and rail but no biggie. |
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Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. A badly installed barrel extension can do the same, though unlikely. 3-5MOA means something is very wrong if you are using good ammo... but it could be a bad barrel there as well, since you arent using a known good barrel. I never recommend using an optic mount and bridging the gap between the upper receiver and a handguard. That is always a bad idea. Why is bridging bad for installation? I don't shoot it this way, but when I install rails that have a same height 12 o clock rail as the receiver, using a LaRue mount across both ensures that the rails are lined up. I have always had success doing it this way in the past. 1. Rails can and will move, at the worst times. Even if it is ever so slight. This will throw off your zero. 2. Look at a gun firing in slo-mo - there is a LOT of flex that occurs and this is bad for the mount, and eventually the zero. I hear you, but like I said, I don't shoot with the scope mount bridging, I just do that for the install to ensure the rails are lined up. |
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Quoted: Quoted: Quoted: Quoted: Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. A badly installed barrel extension can do the same, though unlikely. 3-5MOA means something is very wrong if you are using good ammo... but it could be a bad barrel there as well, since you arent using a known good barrel. I never recommend using an optic mount and bridging the gap between the upper receiver and a handguard. That is always a bad idea. Why is bridging bad for installation? I don't shoot it this way, but when I install rails that have a same height 12 o clock rail as the receiver, using a LaRue mount across both ensures that the rails are lined up. I have always had success doing it this way in the past. 1. Rails can and will move, at the worst times. Even if it is ever so slight. This will throw off your zero. 2. Look at a gun firing in slo-mo - there is a LOT of flex that occurs and this is bad for the mount, and eventually the zero. I hear you, but like I said, I don't shoot with the scope mount bridging, I just do that for the install to ensure the rails are lined up. Oh - I missed that part. Sorry. I do mine by eye. Way more accurate.  |
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i've got a troy rail. when i asembled it i had the same angle up thing going on. i loosened up the pinch bolts and flipped the upper and rail over. set them on a flat surface and with a little pressure on the bottom of the rail and upper retightened the pinch bolts. was straight after that. had a tiny gap between upper and rail but no biggie. That's not a bad idea, but I don't think it will work with mine. I loosened the rail then placed the largest allen wrench I could between the bottom of the gas block and the rail. Then I tightened the pinch bolts all the way and removed the wrench from under the gas block. As soon as I did that, the forearm went right back to hitting the gas block. I think this upper is just not square. In fact, looking at it, it not only is angled up, but it looks to be angled up and to the left. To me, this indicates the receiver is not right. |
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This is the tool I've used to square up receivers with 180 grit abrasive compound with great success.
Make sure the interior of the receiver is spotless and well lubricated. Your drill should be able to turn it freely. I used a low speed setting on my 18v 1/2" drill just fine. 80% of the face needs to be cleaned at a minimum, and it sounds like yours may need all of it trued. Be sure to check headspace afterwards. If this doesn't fix the problem you'd probably want to send it back and tell the MFG to check for true. Good luck. |
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This is the tool I've used to square up receivers with 180 grit abrasive compound with great success. Make sure the interior of the receiver is spotless and well lubricated. Your drill should be able to turn it freely. I used a low speed setting on my 18v 1/2" drill just fine. 80% of the face needs to be cleaned at a minimum, and it sounds like yours may need all of it trued. Be sure to check headspace afterwards. If this doesn't fix the problem you'd probably want to send it back and tell the MFG to check for true. Good luck. Thank's, I've seen that tool before, but after 10 years of putting these things together, I have never ran into a problem where I thought I needed that... but here we are... So today I loosened the forearm again, laid the rifle upside down on its rails as the previous poster mentioned and sure enough, when the forearm is loose it straightens right out leaves plenty of clearance between the forearm and the gas block. Yet as soon as I start to tighten the pinch bolts, it immediately starts tracking upwards again and hit the block. I don't have to have the bolts more than snug for it to naturally hit the block. Does all this indicate upper for sure then? |
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Quoted: This is the tool I've used to square up receivers with 180 grit abrasive compound with great success. Make sure the interior of the receiver is spotless and well lubricated. Your drill should be able to turn it freely. I used a low speed setting on my 18v 1/2" drill just fine. 80% of the face needs to be cleaned at a minimum, and it sounds like yours may need all of it trued. Be sure to check headspace afterwards. If this doesn't fix the problem you'd probably want to send it back and tell the MFG to check for true. Good luck. You are right on the money on the tool to use - but this has zero affect on headspace. |
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Quoted: Thank's, I've seen that tool before, but after 10 years of putting these things together, I have never ran into a problem where I thought I needed that... but here we are... So today I loosened the forearm again, laid the rifle upside down on its rails as the previous poster mentioned and sure enough, when the forearm is loose it straightens right out leaves plenty of clearance between the forearm and the gas block. Yet as soon as I start to tighten the pinch bolts, it immediately starts tracking upwards again and hit the block. I don't have to have the bolts more than snug for it to naturally hit the block. Does all this indicate upper for sure then? No. Technically - it *could* be a defect in the tube itself... however doubtful. Testing on a different upper receiver would prove otherwise. |
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This is the tool I've used to square up receivers with 180 grit abrasive compound with great success. Make sure the interior of the receiver is spotless and well lubricated. Your drill should be able to turn it freely. I used a low speed setting on my 18v 1/2" drill just fine. 80% of the face needs to be cleaned at a minimum, and it sounds like yours may need all of it trued. Be sure to check headspace afterwards. If this doesn't fix the problem you'd probably want to send it back and tell the MFG to check for true. Good luck. You are right on the money on the tool to use - but this has zero affect on headspace. Yah, I was about to say, headspacing is dependent on the bolt interfacing with the barrel extension, which, if the barrel is put together right, shouldn't change if you true up the front of the reciever. |
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Thank's, I've seen that tool before, but after 10 years of putting these things together, I have never ran into a problem where I thought I needed that... but here we are... So today I loosened the forearm again, laid the rifle upside down on its rails as the previous poster mentioned and sure enough, when the forearm is loose it straightens right out leaves plenty of clearance between the forearm and the gas block. Yet as soon as I start to tighten the pinch bolts, it immediately starts tracking upwards again and hit the block. I don't have to have the bolts more than snug for it to naturally hit the block. Does all this indicate upper for sure then? No. Technically - it *could* be a defect in the tube itself... however doubtful. Testing on a different upper receiver would prove otherwise. How about a defect in the barrel nut? This forearm uses a proprietary nut after all... I'm just thinking out loud. I have a new billet match upper on the way and they claim .0003" tolerance thresholds, so I'll try that and see. Eitherway, I am not too supportive of this forearm's design. The pinch bolt system seems too finicky and potentially weak. I'm paranoid everytime I loosen it that I'm going to strip out the helicoils. |
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I had a similar issue with a DD LITE rail a while back and lapping it fixed it. I have since done all my AR's. http://i199.photobucket.com/albums/aa185/heath223/005-16.jpg http://i199.photobucket.com/albums/aa185/heath223/008-13.jpg The pilot of the lapping tool will fit in the receiver with that vice block? That is the same one I have. |
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I had a similar issue with a DD LITE rail a while back and lapping it fixed it. I have since done all my AR's. http://i199.photobucket.com/albums/aa185/heath223/005-16.jpg http://i199.photobucket.com/albums/aa185/heath223/008-13.jpg The pilot of the lapping tool will fit in the receiver with that vice block? That is the same one I have. I doubt that. I made that one
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I had a similar issue with a DD LITE rail a while back and lapping it fixed it. I have since done all my AR's. http://i199.photobucket.com/albums/aa185/heath223/005-16.jpg http://i199.photobucket.com/albums/aa185/heath223/008-13.jpg The pilot of the lapping tool will fit in the receiver with that vice block? That is the same one I have. I doubt that. I made that one Oh, it looks like my DPMS one. |
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I had a similar issue with a DD LITE rail a while back and lapping it fixed it. I have since done all my AR's. http://i199.photobucket.com/albums/aa185/heath223/005-16.jpg http://i199.photobucket.com/albums/aa185/heath223/008-13.jpg The pilot of the lapping tool will fit in the receiver with that vice block? That is the same one I have. I doubt that. I made that one Oh, it looks like my DPMS one. Yea that is something I made just to hold an upper still while I clean it or mount scopes etc... |
| The type of block needed for lapping the receiver would be clamshell, PRI, or Schuster. The Panther Claw block is machined to provide stability at the pins and inside the receiver. Schuster blocks are at AIM for $33 I IIRC. The PRI will also allow aligning of picatinny freefloats also. |
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Quoted: The type of block needed for lapping the receiver would be clamshell, PRI, or Schuster. The Panther Claw block is machined to provide stability at the pins and inside the receiver. Schuster blocks are at AIM for $33 I IIRC. The PRI will also allow aligning of picatinny freefloats also. Or you could use two pieces of free wood, or a chopped up cutting board, like I did:  |
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The type of block needed for lapping the receiver would be clamshell, PRI, or Schuster. The Panther Claw block is machined to provide stability at the pins and inside the receiver. Schuster blocks are at AIM for $33 I IIRC. The PRI will also allow aligning of picatinny freefloats also. Or you could use two pieces of free wood, or a chopped up cutting board, like I did: http://kevinholman.com/byor/ar10noveske/upperclamped.jpg Your putting union workers out of work with this ingenuity. 
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Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. This is were I make the point to my friends when they ridicule me on how expensive my builds can sometimes be. The recommendation MSTN gives is Larue, DD, and Vltor uppers only. Although I would think the source of Noveske uppers, Stags, Mega, and Colt uppers would all be good also. |
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Quoted: Quoted: Upper receivers most often are not perfectly square. This is where using a high quality upper receiver if often a good choice on an accurate build. Some machinists will clean up and true the face of the receiver and ensure it is perpendicular to the bore. This is were I make the point to my friends when they ridicule me on how expensive my builds can sometimes be. The recommendation MSTN gives is Larue, DD, and Vltor uppers only. Although I would think the source of Noveske uppers, Stags, Mega, and Colt uppers would all be good also. Or - if you happen to find one that is off bad enough to cause an issue, lap it with a $35 tool. To date, I never have gotten one bad enough to be an issue. I have seen them, and read about them... but for the ~50 or so uppers I have personally assembled, I have never had an issue (yet). Of course, I would only know it to be an issue if the rifle would not zero, or where a barrel/block was hitting a free float tube. |
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