There is no such thing as a win ultramag so I am assuming you mean the 300rem ultra mag or the 300wsm.Either way neither would work well as a varmint rifle.The recoil is too stiff for a day of varmint shooting and the barrel life would be extremely short for varmint shooting.As the others have already stated; accuracy with accelerators (even if you can find the bullets and sabots to load as they are not available for either cartridge from the factory )is not nearly good enough for varmint shooting.