Hello. New here. I just built a ar9 and for some reason my bcg does not rack. I feel like it’s rubbing on the buffer tube thread. It rack about 1/3 of the way. As you can see, when the takedown pin is out, it fully racks. When the takedown pin is inserted it gets stuck. Any suggestions is highly appreciated. Thanks in advance
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Do you have another buffer tube (receiver extension)?
It sounds as it you may have a buffer tube that isn’t kosher, OR, the receiver extension threads on you lower aren’t 'square with the world.
I’d suggest a new rear take down pin, but 1) if it fits through the holes it must not be screwed up; and 2) it is obvious from your third photo the upper is slightly elevated out of the lower.
I’d also run the buffer and spring back and forth in the receiver extension with a dowel to ensure they aren’t binding, and then do the same thing with the BCG.
Just to be sure.
Somethings out of alignment - the bore of the upper and the receiver extension need to be in line with each other for the bolt to have a straight shot - the upper receiver could be off, the lower’s receiver extension threads might be off, or the buffer tube is off. Drill down to it by the process of elimination.
Do you have another lower to put the upper onto? If the upper works on another lower, then the problem more than likely isn’t the upper.
Maybe the lug on your upper is drilled out of spec?
Maybe the FA switch is bad JU-JU.
Try the lower with another upper.
Try the upper with another lower.
Diagnose the issue
So i tried my upper on another lower and works flawlessly. One thing i did find is when i rack it back, the bolt stays back without me using the bolt catch. Can it be the buffer tube be warped?, I’ve had this lower for quite some time. I dont think it’s my lower but my buffer tube. I dont want to go out buying parts to then figure out it’s nothing i could have done.
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The bolt carrier stays back without an inserted magazine?
As I mentioned in my post, try running the BCG into the tube w/o the upper in place.