Well, prehaps I should explain this further.
It is gona be used for my weapon set. Some weapons are heavier than others. When you mount some, I want to it change the value of the datablock you are using, so you can still jet, or use tacknife.
Thats an example of things not working the way you want.
What will happen is that when one person equips a heavy weapon, and your script changes the datablock to slow down that player - EVERYBODY will slow down. The same datablock is used for all players, so changing the values affects everyone.
Your best option here is to follow picketrocket's advice and make different player datablocks for each weapon type.