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Posted: 5/12/2026 8:34:26 PM EDT
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I got a little excitable 10 years ago and pieced together some parts. The BCM KMR had just come out (magnesium) and I thought I'd build myself a cool kid gun. 14.5" BCM barrel, magnesium rail, pinned BCM comp. I think I had ADCO assemble it. Well, it got a used Aimpoint and sat in the safe for ten years. Prolly got sighted in and shot once or twice after. I had other toys. Flash forward to now and I decide it really isn't a bad setup, I'll just modernize it and go Mlok. For ease, I got the BCM rail so I wouldn't have to fk with the barrel nut. Well, I install it easily and, good gravy, I notice that bitch is way off kilter. Attached File The gun was barely touched, so I know it isn't a bent barrel. I reinstalled the original BCM rail and it's also off. When I think through how the barrel nut and rail are manufactured, I can't imagine they are the problem. So as hesitant as I am to invest time and money into this, I can only surmise that the upper (No forge code, no markings, just T-numbers) is the only culprit. Should the next step be a new upper and reinstall everything? Pisses me off because I only got this rail to avoid wrenching on the upper altogether. I ripped off that rear sight assuming I'd find a Spike's logo, because those douchecocks haven't found a part they couldn't fuck up, but I don't see a logo. Attached File |
"Here we are having a nice discussion on swords and the fucking pikemen gotta shit all over the place." - Silverbulletz06
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Could be the face of the upper where the barrel extension flange sits isn't square. Not super common, but it happens. I had a Stag upper from back around '05 that did the same thing when I went to put a FF rail on it. They make a tool that you can use to lap the face and get it square. Of course make sure the rail is installing straight as well. I always use a block in the vice that clamps onto the pic rail and bridge it across the upper and the top of the rail while I attach it. |
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I would recommend this. The lip the barrel mates up to probably isn’t square and over 16inches of barrel and muzzle device show it being so much off. Wheeler Lapping Tool |
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Originally Posted By backbencher: If it zeroes, who cares, OP? The irons literally can't be sighted on this. I spent the money on BCM parts, LWRC sights, a match trigger. For some reason, this gun has the smoothest bolt pull of any of my carbines. I refuse to have it look tarded. I have a Del Ton upper on the way. That company never gave me drama. Would have sprung for a BCM upper but they're out of stock. |
"Here we are having a nice discussion on swords and the fucking pikemen gotta shit all over the place." - Silverbulletz06
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Originally Posted By BUCC_Guy: Now that I know it's tarded, it bothers me. The irons literally can't be sighted on this. I spent the money on BCM parts, LWRC sights, a match trigger. For some reason, this gun has the smoothest bolt pull of any of my carbines. I refuse to have it look tarded. I have a Del Ton upper on the way. That company never gave me drama. Would have sprung for a BCM upper but they're out of stock. BCM DEMO STRIPPED UPPER RECEIVERS $59.95 (Worth the cost of admission). |
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As mentioned previously... https://www.wheelertools.com/gunsmithing-tools/bore-lapping/ar-receiver-lapping-tool/1082227.html Don't bother attaching a drill, just hand lap the receiver face until the anodizing is removed all around the edge and no more. You'll likely see the anodizing on one side of the face come off as you begin to lap. That high side is what's causing the problem. I've had it happen and fixed it this way. You could just get a new receiver, but then the receiver that's off will just sit in your safe another 10 years or so because ya can't bring yourself to throw it away. At least that's how I tick. Might as well fix it now and get that out of the way. I lap every receiver face since I have the tool. Anytime a barrel gets pulled, the receiver face gets lapped with the tool if it hasn't already been done. |
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Originally Posted By cttb: Have you tried? If the dot zeroed without much windage adjustment, the problem isn't with the way the barrel flange fits against the receiver's face. Originally Posted By cttb: Originally Posted By BUCC_Guy: . . . . The irons literally can't be sighted on this. Have you tried? If the dot zeroed without much windage adjustment, the problem isn't with the way the barrel flange fits against the receiver's face. Given how far off the front rail is, you'd probably need shims front & rear to get it zeroed as it sits. |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Originally Posted By backbencher: Given how far off the front rail is, you'd probably need shims front & rear to get it zeroed as it sits. Originally Posted By backbencher: Originally Posted By cttb: Originally Posted By BUCC_Guy: . . . . The irons literally can't be sighted on this. Have you tried? If the dot zeroed without much windage adjustment, the problem isn't with the way the barrel flange fits against the receiver's face. Given how far off the front rail is, you'd probably need shims front & rear to get it zeroed as it sits. Maybe. To me, it makes a lot more sense to shoot it to see what it's doing before taking it apart and grinding on the receiver. If the dot zeros readily, then the bore is in line with the top rail, which should be the end goal. If my rifle zeros readily and holds it, I don't care if the barrel is off center inside the handguard. |
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Originally Posted By cttb: Maybe. To me, it makes a lot more sense to shoot it to see what it's doing before taking it apart and grinding on the receiver. If the dot zeros readily, then the bore is in line with the top rail, which should be the end goal. If my rifle zeros readily and holds it, I don't care if the barrel is off center inside the handguard. Originally Posted By cttb: Originally Posted By backbencher: Originally Posted By cttb: Originally Posted By BUCC_Guy: . . . . The irons literally can't be sighted on this. Have you tried? If the dot zeroed without much windage adjustment, the problem isn't with the way the barrel flange fits against the receiver's face. Given how far off the front rail is, you'd probably need shims front & rear to get it zeroed as it sits. Maybe. To me, it makes a lot more sense to shoot it to see what it's doing before taking it apart and grinding on the receiver. If the dot zeros readily, then the bore is in line with the top rail, which should be the end goal. If my rifle zeros readily and holds it, I don't care if the barrel is off center inside the handguard. That's fair, but having zeroed a Colt M4 w/ a tilted FSB that wasn't on paper @ 25 from mechanical zero, I think OP's gonna run out of windage adjustment w/ irons. |
I sell firearms produced by the finest child labor in the world, be it Filipino, Muslim, Mormon, Arizonan, or Texan.
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Originally Posted By BUCC_Guy: Now that I know it's tarded, it bothers me. The irons literally can't be sighted on this. I spent the money on BCM parts, LWRC sights, a match trigger. For some reason, this gun has the smoothest bolt pull of any of my carbines. I refuse to have it look tarded. I have a Del Ton upper on the way. That company never gave me drama. Would have sprung for a BCM upper but they're out of stock. Del Ton is closed, is it a used upper? |
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Originally Posted By DoctorCAD: Del Ton is closed, is it a used upper? Originally Posted By DoctorCAD: Originally Posted By BUCC_Guy: Now that I know it's tarded, it bothers me. The irons literally can't be sighted on this. I spent the money on BCM parts, LWRC sights, a match trigger. For some reason, this gun has the smoothest bolt pull of any of my carbines. I refuse to have it look tarded. I have a Del Ton upper on the way. That company never gave me drama. Would have sprung for a BCM upper but they're out of stock. Del Ton is closed, is it a used upper? Looks like old inventory at a vendor. *shrug* |
"Here we are having a nice discussion on swords and the fucking pikemen gotta shit all over the place." - Silverbulletz06
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I just bought one of THESE and it is immaculate. I wouldn't personally be able to stand this either OP. I'm not beating up on anybody else who could, but being that cockeyed would drive me nuts. Even if it shot fine. |
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I'm going to say it again: Step 1 is to shoot the gun. If the dot zeros with typical amounts of windage adjustment, then there is nothing wrong with the receiver. I don't understand buying new parts and disassembling before performing basic troubleshooting. OP: What is the windage adjustment on the dot set at right now? |
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