I am assembling my first lower with an Aero Gen 2 lower, Geissele Trigger and Aero LPK minus FCG, and it all went together well with the exception of when I mate an upper. Function is good but if I pull the stock out semi aggressively the bolt comes off the bolt catch and goes into battery. I have a factory Smith and Wesson lower and I measured the height of the bolt catch when depressed and the Aero Precision is almost 1/10th of an inch shorter. I verified that I have the correct spring as the Geissele comes pre-assembled and the bolt catch buffer is able to go well below the top of the hole. I installed the Aero bolt catch on my SW lower and it will also release the bolt when you pull the stock out to the last position. I am thinking about just filing the bolt catch to get a little more travel. See the picture of the two catches below Aero on top and SW Below

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I ended up filing the bolt catch to allow more engagement on the bolt. We will see what happens.

The bolt is being held against the catch by the buffer springs tension. The catch is being held up by the bolts tension. If the rifle is jarred (as extending a stock can do), it will allow the spring to compress from 'momentum' and release the bolt.