Quoted:OK, I've built a fair number of 9mm ARs now, and I've always used the bolts with the 90-degree key for solid purchase on the charging handle. I don't understand the 45-degree cut keys and how the charging handle can securely lock on them. How the fuck does that work?
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Since all of the components are made of rigid materials (AKA metal) and moving through a confined space, it's not physically possible for the front end of the charging handle to ride up the leading edge of an angled carrier key. And, as previously answered, the charging handle only connects with the lower flat portion of the bolt key to pull the bolt back.