Rate of twist of the rifling is the same regardless of the barrel length. Flatter trajectory would have to do with velocity. High velocity or hyper-velocity ammo would give you more speed but that may not translate into accuracy. One has to try many different loads to see which one will be the most accurate in your pistol. Couldn't recommend a accuracy load since even the same makes of pistol vary enough that it may take different loads for the same make of pistol. The longer barrel will give you higher velocity. Try several brands of highvelocity hollowpoints from Winchester,Federal,CCI,and Remintington. Shoot 3 or 5 shot groups to see which brand has the tightest groups. Do this several times. Let the barrel cool every few shots. Hot barrels can distort groups. Others will have good advice to follow.