AR Sponsor
Posted: 5/10/2015 8:43:04 PM EDT
Hello I was having a heck of a time zeroing the AR. Then noticed that when looking down the upper the FSB looked canted to the left, but its all point of view it sometimes looks fine and then it looks canted... I took a level and set it on top of the FSB and it was level so doesnt that mean its fine?
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How loose does the barrel fit inside of the upper receiver, as it shouldn't have a lot of wiggle room the the point of being sloppy loose? If there is a little play of how the barrel extension indexing pin fits in the upper receiver (this is the pin that sits up top dead center that fits inside the milled slot in the top front of the upper receiver), you could shim as long as it doesn't cause the barrel to rotate too much and cause a feeding issue. Sometimes a front sight base can also be mounted off center as little. Another possible issue may be that the front face of the receiver may be off square, and if that is the case then it can be dusted off a little by a machinist. Also if checking with bubble level, you might put the bubble level on top of the flat top of the upper receiver, as the bottom rear take down lug may not be machined flat on the bottom to be in alignment with the top of the receiver. Any chance you can provide the brand of the barrel and upper receiver?
CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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Quoted:
How loose does the barrel fit inside of the upper receiver, as it shouldn't have a lot of wiggle room the the point of being sloppy loose? If there is a little play of how the barrel extension indexing pin fits in the upper receiver (this is the pin that sits up top dead center that fits inside the milled slot in the top front of the upper receiver), you could shim as long as it doesn't cause the barrel to rotate too much and cause a feeding issue. Sometimes a front sight base can also be mounted off center as little. Another possible issue may be that the front face of the receiver may be off square, and if that is the case then it can be dusted off a little by a machinist. Also if checking with bubble level, you might put the bubble level on top of the flat top of the upper receiver, as the bottom rear take down lug may not be machined flat on the bottom to be in alignment with the top of the receiver. Any chance you can provide the brand of the barrel and upper receiver? CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 There is free play but not a lot.. You can move the barrel side to side and hear a little click (from hitting the side of the slot) and it seems to have a slight influence on the FSB position. I checked the level on the flat top, it is basically level with the bubble not perfectly in the middle but close enough (could be the surface im on). The barrel is a Colt 14.5" SOCOM and the upper is a BCM demo / scratched M4 flattop. |
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There is free play but not a lot.. You can move the barrel side to side and hear a little click (from hitting the side of the slot) and it seems to have a slight influence on the FSB position. I checked the level on the flat top, it is basically level with the bubble not perfectly in the middle but close enough (could be the surface im on). The barrel is a Colt 14.5" SOCOM and the upper is a BCM demo / scratched M4 flattop. Quoted:
Quoted:
How loose does the barrel fit inside of the upper receiver, as it shouldn't have a lot of wiggle room the the point of being sloppy loose? If there is a little play of how the barrel extension indexing pin fits in the upper receiver (this is the pin that sits up top dead center that fits inside the milled slot in the top front of the upper receiver), you could shim as long as it doesn't cause the barrel to rotate too much and cause a feeding issue. Sometimes a front sight base can also be mounted off center as little. Another possible issue may be that the front face of the receiver may be off square, and if that is the case then it can be dusted off a little by a machinist. Also if checking with bubble level, you might put the bubble level on top of the flat top of the upper receiver, as the bottom rear take down lug may not be machined flat on the bottom to be in alignment with the top of the receiver. Any chance you can provide the brand of the barrel and upper receiver? CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 There is free play but not a lot.. You can move the barrel side to side and hear a little click (from hitting the side of the slot) and it seems to have a slight influence on the FSB position. I checked the level on the flat top, it is basically level with the bubble not perfectly in the middle but close enough (could be the surface im on). The barrel is a Colt 14.5" SOCOM and the upper is a BCM demo / scratched M4 flattop. Free play is not normal. What torque setting did you use for your barrel nut? |
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Quoted:
? Free play is not normal. What torque setting did you use for your barrel nut? Quoted:
? Quoted:
Quoted:
How loose does the barrel fit inside of the upper receiver, as it shouldn't have a lot of wiggle room the the point of being sloppy loose? If there is a little play of how the barrel extension indexing pin fits in the upper receiver (this is the pin that sits up top dead center that fits inside the milled slot in the top front of the upper receiver), you could shim as long as it doesn't cause the barrel to rotate too much and cause a feeding issue. Sometimes a front sight base can also be mounted off center as little. Another possible issue may be that the front face of the receiver may be off square, and if that is the case then it can be dusted off a little by a machinist. Also if checking with bubble level, you might put the bubble level on top of the flat top of the upper receiver, as the bottom rear take down lug may not be machined flat on the bottom to be in alignment with the top of the receiver. Any chance you can provide the brand of the barrel and upper receiver? CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 There is free play but not a lot.. You can move the barrel side to side and hear a little click (from hitting the side of the slot) and it seems to have a slight influence on the FSB position. I checked the level on the flat top, it is basically level with the bubble not perfectly in the middle but close enough (could be the surface im on). The barrel is a Colt 14.5" SOCOM and the upper is a BCM demo / scratched M4 flattop. Free play is not normal. What torque setting did you use for your barrel nut? I cannot remember what I torqued it to.. I probably over torqued it trying to align the gas tube so I think I will order up a new upper receiver and see if that fixes it. |
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Something doesn't sound quite right. If the barrel is tightened enough(??) there should be NO wiggle of the barrel in the upper. ZERO NADDA You generally should be able to tighten the barrel nut with hand pressure enough to prevent much if any looseness.
How snug was the barrel fit(extension) inside the upper receiver? Assume the barrel came with the FSB already installed? |
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Quoted:
Something doesn't sound quite right. If the barrel is tightened enough(??) there should be NO wiggle of the barrel in the upper. ZERO NADDA You generally should be able to tighten the barrel nut with hand pressure enough to prevent much if any looseness. How snug was the barrel fit(extension) inside the upper receiver? Assume the barrel came with the FSB already installed? The extension is quite snug, you cant just pull the barrel out of the receiver. I think I messed up the slot in the receiver.. With the barrel nut hand tightened I can still move the barrel side to side (Makes click sound from hitting the side of the slot) its very minimal and barely enough to notice but it does move some. The barrel was bought separately from the upper receiver. 
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OP, the barrel nut should be torqued to 30 ft-lbs. Correct procedure is to torque, loosen, and re-torque three times to seat the barrel and prevent looseness. The barrel nut is then aligned for the gas tube either with a #15 brill bit, or the gas tube. When inserted, correct alignment is achieved when either bit or tube points directly down the center of the barrel. The upper limit for torque is 80 ft-lbs.
There's a procedure in the Army Training Manual for centering the front sight base. 
This is done with the upper receiver assembly secured with a barrel vise, and the bolt carrier group removed. You'll need a rear iron sight installed. As the TM page says, when the front sight post is correctly aligned, the FSP should be in the center of the rear sight's aperture, with the rear sight centered. At lower barrel nut torque settings, you can usually move the barrel a little by inserting a long screwdriver through the front sight base, and applying torque in the needed direction (take care to not damage the FSP). You should be able to get it in the ballpark with this method, but barrels misaligned at the time of FSP pinning are not unheard of. The spec for maximum windage adjustment is thirteen clicks either way: if you can zero within that range, it's good to go. HTH. 
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Your looking for two things here,
The barrel bore mounted so it's in line with the center line of the receiver, and then the FSB mounted level with the TDC of the upper receiver. So if you have the barrel nut mounted correctly (plus 30ftlb, but not more that 80ftlbs), then level on the top rail ,and level on the FSB for starters. Next loop some string around the FSB post, and pull it back to the center back of the top rail. Now look down the upper center rail, and see if the string at the front of the rail is center line as well (straight edge/ laser down the upper top rail to the center line of the FSB will work as well), of if the barrel is canted off to one side of the upper receiver center line. Now lets say that the barrel and it bore is cocked down one side of the upper receiver bore line. Step one, pull the barrel and spin it in a lathe between live center (centers on the chamber entry and muzzle exit) to make sure that the barrel is is not bent, and the barrel extension was installed true to the barrel bore to begin with. Here on the barrel extension, just use a dial indicator to make sure that the OD of the barrel extension and back lip face are running true with the bore of the straight barrel. If need, then back lip of the barrel extension back be cleaned up a touch, so long as the main OD of the barrel extension is not way out instead. Step two, check the channel and front lip of the upper receiver barrel socket to make sure that it was milled true with the upper receiver center line. Here, if too much torque is used to cant the socket off center line, the barrel socket in not true to begin with, or you have debris between the back face of the barrel extension lip to face of barrel socket, these will all cause the barrel be canted off the of the upper receiver center line when the barrel is mounted. As for if the upper receiver barrel socket needs to be trued, couple ways of resolving that, and I prefer Mandel pressed in the receiver to first true the barrel socket channel to begin with to the center line of the upper receiver, then face the front of the socket last to the upper receiver center line. 
Upper receiver socket channel set back on center line of upper receiver, front face trued with the upper receiver center line, and ready for the barrel to be reinstalled. 
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I'm thinking my FSB is canted as well. Ran out of windage on the rear sight before I could get it zeroed. Anyone experience this? Pretty easy to slip the entire barrel in the upper receiver barrel socket if the problem is just the FSB is slightly canted off to the one side; while the bore is true with the upper receiver itself. When the barrel nut is tighten, it can cause the entire barrel to slip in the receiver socket to cause the FSB to be canted off to one side, so slipping the entire barrel back in the socket with a tapper pinned FSB is a snap. So the question does down to if the barrel just needs to be slipped in the upper receiver barrel socket with the barrel nut still tight to index the FSB correctly, or if the barrel itself is not true with the upper receiver bore instead. Most of the time, it just a matter of slipping the barrel in the upper receiver socket to correct the problem. 
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