I'll second what wahoo95 said about lubricating the pins / holes prior to installation. It makes a world of difference when installing the roll pins. That said, the bolt catch roll pin sometimes is a PITA. I recommend using a roll-pin starter punch here as it really helps out and virtually negates the possibility of scratching your lower.
Also, ensure to properly grease the barrel nut threads / receiver threads and where the barrel nut contacts to flat portion of the barrel extension (see picture below). A friend tried to install a barrel without adding any grease and was completely flustered as to why he could not line up the gas port without excessive torque.
Take your time and use the proper tools.
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