How common is it for an upper not to fit a lower?

[WS6]

Your lower is right on the bottom edge of the spec. The lower receiver blueprints call for .500" +0.000/-.004 It is likely that it will be problematic with many uppers. I've yet to find a blueprint for uppers so I can figure out what their specs are. (ETA: I have found upper prints, but none of them have the dimension for the mating surfaces between the upper and lower) I would imagine that there is some point within the tolerances that a upper on the big side of spec won't fit in a lower that is on the small side of spec.

That is my conclusion as well. I do not expect the company which makes the upper to run around their place mic'ing uppers. I am going to just ask for a refund, slap the former (that fits) 10.5" upper back on it. Hopefully the company will work with me. I am pretty sure they will, they are one of the best around, especially on the CS department. No names, as I have not talked to them yet, this is not a "bash thread", etc.

[jonconsiglio]

Like I said above, I wouldn't hesitate to two the pin through, if possible. Doing this a couple times might get you where you want to be.

[WS6]

Like I said above, I wouldn't hesitate to two the pin through, if possible. Doing this a couple times might get you where you want to be.

I don't want to break anything. It feels like there is a good chance of that if I muscle it.

[markm]

Reminds me of the old Olympic Arms days.

[WS6]

Reminds me of the old Olympic Arms days.

Lol, no, the upper and lower are both by some of the best companies out there, by reputation.

[Moltke]

I have a very tight fitting upper/lower from different manufacturers... but they do fit and the gun does work.

[WS6]

I have contacted the manufacturer of the upper and a solution is in the works. So far I am very pleased, and when there is resolution, I will post up the names and what happened. I am refraining from this thus far as this is not a fanboy or a bash thread at this point, as nothing has ACTUALLY been done, but the promises are very promising.

[markm]

Lol, no, the upper and lower are both by some of the best companies out there, by reputation.

I hear you. That issue used to belong to Oly though.

[Failure2Stop]

I have a very tightly fitting LMT lower and DD upper.
No functional issues, and it sure does make the "upper/lower play" crowd happy.
I only need to break it down for detailed cleaning while I am home anyway so I really don't care that I need to use a punch to drift the pin.

[jonconsiglio]

I don't want to break anything. It feels like there is a good chance of that if I muscle it.

You won't. Remove the buffer, action spring and bolt carrier group and hold it together tight. Use a rubber ma,let or something similar to tap the take down pin. You can hit on it pretty good, but I wouldn't pound it.

Doing this a couple times should loosen it up. My AAC was so tight at first, I was worried and I couldn't try it on anther lower I had since it wasn't SBR'd. But now I can do it by hand like any other upper. I think it was anodizing, but I have no clue.

If that doesn't work, then send it back. Seriously, it'll take a lot to break anything.