Not sure the 14 twist will stabilize the 50 grain Barnes varmint grenade, but if they will stabilize that would be my choice of bullets produced by the major manufacturers. I have killed several coyotes with the Hornady NTX and some with the Nosler Lead Free. Both of them get the job done, but I did have several that ran a ways with shots to the heart/lung area. Seems like the bullets don't deliver the "shock" or energy dump that the heavier lead core bullets do, IMO.
That being said, there are some of the smaller bullets makers that are making lead free mono-metal bullets that have very good reputations for on game performance. I have no expereince with them, but Hammer and Cutting Edge are two that come to mind. They might make something that would be short enough to stabilize in your 14 twists.