The 44 magnum would be the way to go. Shoot the lighter bullets for everything else, and use 300 gr jacketed for the 200 meter targets. Cast or plated bullets won't have enough umph to reliably knock over the rams. 357 will be marginal, and will ring about 1/2 of the rams, at least at my range. The rams are full-sized, and set full-footed, not just forward of the topple zone, like BB pistol silo. B_Koes, what load are you using for your rams to get such reliable knockdowns?
Downside of the 44: pretty stiff recoil, esp with a steel crescent buttplate. PAST or some other recoil shield highly recommended.
Eric