Hi, I'm new here.  I have a build question.  I have folded a flat and am ready to rivet it together.  My question is, when the time comes for fitting the bolt what is the best way to cut down the top rails?  Thanks.

put the bolt assembly in and work the action.  If you notice any binding occurring, trim the top rails.  That's how I atleast do it on my builds.

Most guys use a Dremel tool to trim top rails.  Here is what I do to get the inside dimension between the top rails correct.  I take my dial caliper and measure the width of the bolt carrier in the grooves that fit the top rail then trim to that width.  I know there are lots of places that tell us exactly what measurements to use, but I am a huge fan of measuring the actual parts that  I have and making them fit.  

Thanks for the replies.  My plan was to go with a file first time around.  Not sure how good I would do with a Dremel.