Best stripped lower for first build

It appears you have a poorly finished mil-spec tube which did not have the threads milled down. It's obvious that you know that but the symptoms you are experiencing are usually reserved for lower tier companies like ATI. I have seen owners use thicker cut to fit two way tape on the end of the tube to impede the travel to keep it from jamming.

AR15.com has a few posters who have experienced the identical issues but some so severe the owner can not pull the stock off of the threads at all.

Personally I would change to another tube manufacturer which has the exposed threads properly milled. The attachment illustrates a properly finished mil-spec tube.

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If I had to jam it shut it would probably need a pry bar to open. I did measure my tube against that drawing and all my measurements are within spec but like you said the threads don't taper soon enough at the end for the stock to slide over. I could just throw it on a lathe and take some material off the last few threads, or save it for another time/stock that will fit.

I find it weird that an Aero part is causing this issue given the praise of their receivers.
 
A couple of pictures with two mil-spec tubes and Magpul stocks. One with the visible threads and the second collapsed.

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Thanks Sgr, those pictures definitely define it. With that said, where/what would be the extension tube to buy with properly milled threads so this doesn't happen again? I wouldn't want to buy another milspec tube from another reputable manufacturer and have the same issue again. Thanks!
 
Thanks Sgr, those pictures definitely define it. With that said, where/what would be the extension tube to buy with properly milled threads so this doesn't happen again? I wouldn't want to buy another milspec tube from another reputable manufacturer and have the same issue again. Thanks!

Any particular reason you prefer the Mil-spec over the Commercial versions as the later tube and thread diameters are identical? Also the Commercial Tube is longer and has more positions available then a mil-sec item.
 
Mainly because my CTR stock for this build is milspec. If I went commercial I would have to purchase another stock as well.
 
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