Bounties can be for a certain Battle or an ongoing bounty until you remove it. You may not drop a bounty during a battle, but you may offer one up. Bounties are for the elimination of a mech not for the killing of the mechwarrior.
You may offer up a killing bounty on the mechwarrior, but your mechwarrior will take a -20 Fame hit.
You may make side bets with another player if you wish. First to drop an enemy, Who lasts the longest, First to score a hit with a physical attack, etc.
Current Mech Bounties:
Current Mechwarrior Bounties: