Weightlifting helps a little, but the basics of good powerfull punching lies in good technique.
Boxing is all about how quick you can hit, with the right techniques. Have that, and you're deadly dangerous in the ring. being able to move your arms swiftly has nothing to do with how much weight you can lift.
Taking that in consideration, the fact that with punching a straight right or left, you must strech out your arm to make it straight and land your punches correctly. Weightlifting has the nasty habit of making your muscles short.
To make a comparison: Bodybuilders have short, thick muscles, longdistancerunners have long thin muscles, but both are trained muscles. Boxing keeps the middle of both. You need strong muscles who can strech. So a bit of weightlifting for the overall strength is good, that's why you don't need to lift as much lbs as you can.
Besides: it's more effectivly to do repeating movements with lbs that you can lift rather easely, you get more out of it (incl. muscle-stamina) than doing just a couple of real heavy lifting. That does absolutely nothing for you.
Again a comparison: no marathonrunner trains running the 42km a couple of times a week. He only runs that distance when he's running the marathon, in training he does no more then say 25-30 km.
Anyways, guess that's the reason why you don't see what you expected in those 30 minutes
Hope i helped you out a bit...
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