WEIGHTED BASEBALLS: DO's and DON'T's BASED ON THE RESEARCH
Oh boy, here we go again with all the weighted baseball stuff…
But, seriously, are you educated yet? Probably not. There is so much more research needed to fully understand everything there is about weighted baseballs.
So far, here is some of what we do know in a quick summary:
Using an array of weighted baseballs (4-12oz.) can lead to an increase in throwing velocity.
One study  found that using over-weighted baseballs as a warm up greatly improved both throwing velocity and accuracy.
These results are great, but they lack ecological validity (carry-over effect). Will a pitcher ever warm-up with a non-regulation ball between innings? Probably not.
According to the research, it seems that the most optimal range for progressively increasing intensity is 4-6 ounces. In a multitude of studies by Coop DeRenne [4-5], increasing by 0.25 ounces per week has shown to increase throwing velocity by 6%.
If you plan on implementing a weighted baseball program, the suggested ratio of weighted (over- and under-weighted) baseballs to regulation baseballs is 2:1 . For example, 10 throws with a regulation ball will yield 20 throws with an over- and under-weighted baseball.
Heavier baseballs increase the amount of external rotation of the throwing arm in the arm cocking position, as well as the amount of internal rotation force on the throwing shoulder . External rotation has been linked to an increase in throwing velocity.
Baseballs that are significantly greater (14-oz.) than regulation baseballs (5-oz.) should NOT be used for enhancing throwing mechanics. Increasing arm strength can only occur in the weight room!