When Orcish Bowmasters enters the battlefield, it triggers, creating a 0/0 Orc Army token if you do not control an Army, then putting a +1/+1 counter on an Army you control and dealing 1 damage to any target.
Activate the Orc Army using Magus of the Jar's ability by tapping and sacrificing it, exiling each player's hand and causing each player to draw seven cards.
Orcish Bowmasters triggers for each card opponents draw this way.
Resolve the first trigger, creating a 0/0 Orc Army token, then putting a +1/+1 counter on it and dealing 1 damage to any target.
Holding priority, repeat from step 3 until you have six or less cards in your library.
Resolve the first remaining Orcish Bowmasters trigger, creating a 0/0 Orc Army token if you do not control one, then putting a +1/+1 counter on an Army you control and dealing 1 damage to any target.
Repeat from step 7 for each remaining trigger.
At the beginning of your end step, each Orc Army that used Magus of the Jar's ability triggers, returning all cards exiled with them to their owner's hands.