The resistance due to the increased torque forces imparted to the barrel by the bullet in the M&P helps to prolong the dwell time. Think of it as trying to twist and pull on the barrel while firing the pistol. The faster twist increases the rotational forces on the barrel which, in a sense binds the barrel in the slide for a slightly longer period of time. The slower the twist, the less binding effect. It is a minimal gain, however our test results at 50 yards between factory 1:18.75, 1:10, a 1:16 and our 1:10 show statistical relevance to the idea.
The other problem is the wide variation in the slide's muzzle end barrel cut. The bore dimensions on the front end of the slide vary wildly, and the relief cut so that you can remove and install the barrel also varies by several thousandths. We know that as soon as you fire a shot, the gun with a factory or even aftermarket barrel loses uniform lock up stability. This is due to how the engineers designed the barrel locking surfaces. The looser the contact between the front of the slide and the bottom of the muzzle due to a larger bore or smaller barrel OD exacerbates the problem. This is in part, why some guns will show improvement with aftermarket barrels while others will not. The vertical slide to frame tolerances also contribute to the instability of the barrel because the bottom lug of the barrel never really makes contact with the locking block until the barrel is moving down and to the rear under recoil.
In order to achieve consistent accuracy the bottom lug should make contact with the horizontal surface of the locking block, and the front of the barrel needs to be as close to the slide bore dimension as possible.
