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Posted: 1/10/2009 7:17:14 PM EDT
| Ok ... So I have a flat top receiver that had a 20" dpms barrel in it. It was fine and all was good. I recently ordered a new barrel from bushmaster that is a 16" m4 profile barrel. It got here this morning and I was going to put it together ... BUT the barrel extension pin (the bump on top of the barrel extension) that fits in the slot on the receiver is slightly bigger on this barrel. It won't fit in the receiver. Has anyone seen this before? |
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While I personally haven't seen it before, there are a couple of fixes for you.
1. Buy a new upper (with or without M4 feedramps depending on what your barrel has) and you may not have the same problem. 2. With a fine file, remove just enough material evenly from both sides of your current upper to allow the new pin to fit. |
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I'd be more inclined to kiss the pin on the barrel extension than file the upper.
Granted, uppers are cheaper than barrels - but the barrel is the replaceable wear item more often than not. Whatever you do, be sure to kiss both sides of either the pin or the notch in the upper. Don't know what the exact size of the pin and notch is supposed to be, but I have never seen one before that didn't fit pretty nicely. ETA: If you haven't burned up the DPMS 20" and still want it to fit again, file the barrel extension pin. |
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I have noticed many times that the better barrel makers like WOA have a slightly larger index pin that sometimes requires a bit of work to almost "press" the barrel into place. I have never damage an upper by doing this.
It could also be that if the old 20" was fired a lot, there could have been buildup around a pin that had play around it. I personally would much rather tweak a $90 receiver (especially a DPMS) than do ANYTHING to the index pin on a $250 barrel. |
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Quoted:
I have noticed many times that the better barrel makers like WOA have a slightly larger index pin that sometimes requires a bit of work to almost "press" the barrel into place. I have never damage an upper by doing this. It could also be that if the old 20" was fired a lot, there could have been buildup around a pin that had play around it. I personally would much rather tweak a $90 receiver (especially a DPMS) than do ANYTHING to the index pin on a $250 barrel.[/div] Even though the barrel will be toast before the receiver (in most cases), and filing the receiver will not make it fit well with the next barrel without shimming? |
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Quoted:
2. With a fine file, remove just enough material evenly from both sides of your current upper to allow the new pin to fit. This. It will not take much. Try get it as even as possible or you could end up making quite a bit of a windage adjustment when zeroing it (assuming the FSB is indexed correctly). |
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Quoted:
Quoted:
2. With a fine file, remove just enough material evenly from both sides of your current upper to allow the new pin to fit. This. It will not take much. Try get it as even as possible or you could end up making quite a bit of a windage adjustment when zeroing it (assuming the FSB is indexed correctly). I would not worry aout "windage" as much as loosing the timing of the bolt locking lugs to the bolt if you file one side of the pin or the UR slot more than the other. Why not send the barrel back?. Quite frankly, I don't understand how anyone, even Bushmaster, could screw up what has been a .125" (1/8") pin that has been used for decades and decades on millions of rifles? However, if I was the guy in LEGEND, and therefore without any other possible choice, I'd would only file the front end of the slot (evenly on both sides like a "guide-way"), just enough for the pin to enter. Then I'd depend on the barrel nut torque to set it back in the bottom and center of the slot. I mean the slot is aluminum and the pin (should be?) steel. |
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