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.
http://farm9.staticflickr.com/8077/8...b8d265df_o.jpg
Take your time and use the proper tools.