Ordered the 10# and 30# slam ball with tread - Loved my 20# slam ball and wanted additional sizes. 20# ball is perfect diameter to weight ratio. Filled solidly but zero bounce when slammed. 10# ball is the same diameter as the 20# so can feel the slug of sand moving around inside. Still works, but annoying. 30# is way bigger than the 20# and has the same issue. I'd imagine that the same diameter ball is used for the 40-50#. They feel durable and i'm generally happy but would put more effort looking elsewhere for the 10, 30# balls before buying. These are the first things from Titan that i've ever been even remotely unhappy with. Items arrived solidy packaged and in great condition. Shipping was timely.