Grant, Who do you feel offers more consistent or better lowers/uppers? I personally like mega products, but they are hard to come by right now.
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And the flood of garbage into the AR world has officially begun. :(
According to some drawings, the recess the rear lug of the upper fits in should be 0.500 + .000/- .004 wide, 0.950 long, and 0.630 deep.
Might give you a better idea what's out of spec. I've got 3 rifles built with AP uppers and they all fit well.
Before filing or defiling anything, check the RE and check the buffer & BCG for interference. On my ARs, sometimes the upper & lower cannot be closed without the BCG hitting the buffer.
Also make sure the pin isn't sticking in too far & causing interference
Unless my calipers are off:
Width
http://img.tapatalk.com/d/13/02/14/yzy9utyj.jpg
Lower width
http://img.tapatalk.com/d/13/02/14/qyra6u2a.jpg
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Before setting mind to purpose, is there anything I should keep in mind? Should I contact Aero first and see what they say?
As for the metal, is there anything I can apply to the receiver once it has been filed?
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so .003 off. I would chalk it up to tolerance stacking and not junk parts like so many like to claim right off the bat. If it was .03 off than I would suspect junk parts, but .003, tolerance stacking IMO. Probably one swipe with a file on each side will get it to fit.
Although if what LeviTX posts as the "spec" for the inside of the rear pin area than both parts are WAY off, but I find it hard to believe that both parts would be .05 off from the spec.
ETA: before you blame Aero do you have other uppers you can check? Or other lowers? I never thought Aero turned out junk, but I would be more suspect of the lower since I personally have never heard of Delaware Machine.