What kind of primers do they us in their .223 rounds.  Would the .223 69gr.  Sierra Matchkings they sell be a good bet for 200 and 300 yards shots out of a 1:9 twist 20" bbl?

I would think they would perform very well in the ranges listed. Of course their is no way of saying for sure as different rifles will many times give different results with the same ammo.

-CH

  The .69 grain SIERRA Match King are perfect for the 1/9 twist at 200/300 yd.line.  
  Your 20" AR should handle the .68 gr. Hornady or the .69 gr Sierra's with flying colors.  
  Until just recently, the 69 SMK was THE round to use at Camp Perry on the 200 and 300 yard lines.  
  Most shooters now use the heavier .75gr. Hornady's, .77 gr. SIERRA's or the .80 gr. Bergers especially for the 600 yard line.
  This is what the shooters use at the National Matches at Camp Perry recently.
  Anything more than a .69gr in a 1/9 twist is a Hit or Miss.
Good luck!
JD