Congrats on the M&P 15 Sport. I bought an early one with the A2 type iron sight in 2011. When I sighted at 50yds my x group rounds were touching.
I used it in IDPA 3 gun and it was awesome, fast and accurate from in close out to 200yds.
1/8 twist does good with 55gr bullets but they love 62`s.
Have fun!
Jim