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I use the plugs to measure my chambers as well (on bolt action rifles) and trim the brass to .003" short of an average reading.
Does it do anything good? Maybe, maybe not. Does it do anything bad? Maybe, maybe not.
What it does do is give me the ability to uniform my brass with the longest possible neck for my particular chamber. That means I have the most possible contact between the brass and projectile, and for my thought process that is a good thing.
BTW, the .223 ones are poorly manufactured in my opinion. The plug end is much too thick and needs to be turned down so that it can function properly.
I wouldn't waste my time measuring the chamber in an milspec AR though - they are mass produced barrels that are designed to tolerances that must accept ammo made from umpteen different manufacturers with umpteen different sets of quality control standards. Yes, theoretically they are all held to SAAMI specifications, but we all know and live in the world of reality and should understand that even with grandfather specs, each manufacturer produces ammo that has significant (in reloading terms anyway) dimensional differences.
AR chambers, outside of match or Wylde chambers, are very forgiving for ammo that is not exactly per spec. One of the reasons they are as good of battle rifle platforms as they are.
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