30mm tube, yeah. That's the tube size. Your current scope has a 1" tube but a 20mm objective. The Weaver has a 30mm tube and a 24mm objective.
So, the Weaver is a 1-5x24, your current scope is a 1-4x20.
The Weaver should have considerably better light gathering.
30mm is 1.18 inches. The area of a 1" tube is .7853 sq/inches. The area of a 30mm tube is 1.093 sq/inches. So the 30mm tube is almost 40% bigger.
A 20mm is objective is .787 sq.inches, while a 24mm is .944 sq/inches. The 20mm is .486 sq/inches and the 24 is .700 inches, or 44% bigger.
Curious that the scope would have an objective smaller than the tube. I've never noticed one like that before but I pay no never mind to such small scopes.
(You'd need new rings too, for a 30mm tube.)
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.