I have the same upper you are looking at. I have only used the 69 grain SMK's so far. I have zero complaints about the accuracy. I haven't tried the 55 grain bullets in mine yet. Until I use up the 1,000 plus left over 69 SMK's I don't think I will dip into my stock of 52 grain WATSON's. Mine has a Kriger barrel, and sure shoots.

Thus saith thr lord; Cursed be the man that trusteth in man, and maketh flesh his arm, and whose heart departeh from the lord. Jeremiah 17:5 KJV