Really? I have stacks of them. :D
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I dont have any of their current production stuff, but I have 2 lowers from around 3 years ago and have no problems with them. Now their current business practices they can go **** themselves with a long sharp stick and I will never own another one of their products, but thats another topic for another day.
My only point really I guess was before we throw Aero under the bus(which may be warranted) there are lots of other things at play. Me personally I would be suspect of the lower first since it is, to me, a no name lower from some local company. To me .003 off from each other could account for tolerance stacking.
For sale? I'm saying those brands are hard to find for sale right now. :D
I understand what you are saying though as I will buy BCM, COLT, NOVESKE and DD parts without question since I know they have a solid rep and BCM is one of my favorite parts sources. Other than BCM though, I have trouble finding one brand that has all parts I like. For example, I prefer LMT RE's over BCM, but I don't care much for many of the LMT complete setups. I prefer BCM BCG's over all others. I like BCM and Colt CH's. Etc.
But for those still trying to put gear together or build rifles, sometimes you just can't get those name brands easily or affordable when it comes to uppers or lowers, regardless of how much prior gear one acquired before December.
Since you didn't mention them, what do you think about Mega? Any prior issues you've seen there?
In reference to the original topic, here is another perspective on Aero Upper and Lower:
Receiver extension 1 turn out, the upper closes properly
http://txbonds.zenfolio.com/img/s8/v...30374664-4.jpg
http://txbonds.zenfolio.com/img/s8/v...30374500-4.jpg
Receiver extension all the way in, the way I prefer it, and upper won't close completely
http://txbonds.zenfolio.com/img/s11/...30386822-4.jpg
http://txbonds.zenfolio.com/img/s2/v...30386936-4.jpg
Here is same LMT RE all the way in on a Rainier lower
http://txbonds.zenfolio.com/img/s3/v...30394896-4.jpg
The receiver extension is an LMT and it fits perfectly in my Rainier Arms lower (mega) with mega upper when all the way in. Maybe I'm putting the receiver in further than I should but it shows a difference between the two lowers when one lets me and one doesn't. Out of interest, when putting the mega upper on the Aero lower with receiver extension all the way in, it won't even close this far. (not staked because these are new in progress)
The idea of tolerance stacking needs to go away. When it comes to mating parts, tolerances are set to guarantee proper fit. Take a shaft and a hole, for example. Lets say each is 1in nominal diameter and you want to guarantee a clearance of at least 0.001 but no more than 0.005 you would set the hole tolerance at 1.001in -0, +0.002 and the shaft at 1.000in -0.002, +0. The only way an interference fit can happen is if one of the parts is out of spec.
You don't know what tolerances each manufacturer is holding their parts to and if they allow greater variation than the original design then you can end up with an interference fit. Either the upper or lower was made according to looser tolerances or one of them is out of spec.
Measured the upper receiver takedown pin lugs and the slots in the lower receiver on my BCM LW and Colt 6920 carbines.
BCM Lug Width: .493"
BCM Slot Width: .501"
Colt Lug Width: .494"
Colt Slot Width: .503"
Appears the OP's Lug width is a tad wider and the slot a tad narrower. Couldn't find the exact dimensions and tolerances. There's a saying here on M4C.net about always modify the cheaper part! :)