I have a Caldwell Rock and with front bag it weighs roughly 15lbs , not sure what kind of weight is suppose to be needed but this one stays put once set in place. Evidently you are considering a lighter version of the Rock since you mention your Hart weighing the same?
BUT , as the others mentioned , it is not an elite rest by any means. And for the $119 I paid I did not it exepct it to be. The other rest I was considering was the Sinclair which was about $328 at the time.
My main complaint with my Rock is the top , the controls lack any kind of precision. Will upgrade that when I can. There is a reason it is $200+ less than the Sinclair.