1/18 is the standard Smith 9mm twist in most of their firearms. we feel we get a good compromise for most bullet weights at 1/16....although with the Shields shorter barrel the difference in accuracy is honestly negligible for what the pistol is designed for. We did see an obvious improvement using our Ransom rest with the 1/16 over the 1/18 factory with the commonly used 115 and 124gr ammo. We feel like we can fine tune the rest and get things even tighter. We also wanted to do an in depth look at a few brands of 147gr and see how they compare. We believe they will show improvement over the factory barrel as well.