Tim's reason for taking the brown bess from Sam is two-fold. First, he takes the gun in the hopes of preventing Sam from joining the militia (having a weapon was a requirement for joining). He wants to prevent Sam from being in harm's way and to force Sam to come back home and stop arguing with their father. Second, Tim knows that the brown bess is necessary for his family's survival. Mr. Meeker needs it for both protection and to obtain food. Without the brown bess the rest of the Meeker family will suffer.
Tim takes the brownbess from same because he wants to make thefighting between his father and Sam. Tim also knows that stealing is bad. The brown bess is very important to the family too because the family hunts animals.