This article is aimed at all the Warmachine factions (except Convergence of Cyriss who don't have one) and mainly newer players - but veterans might pick up something here too.
(Ignore Sturgis... this was the best image I could find of all the Jr's together!)
They are not Warcasters
Even though they share a lot of rules, they are not Warcasters.
This means they can't dominate scenario elements, or be affected by effects that work on "friendly Warcaster" such as Reinholdt's reload ability or Gastone benefiting from Rocinante's "guard dog" ability.
They have to start the game with at least one Warjack under their control (unlike Jack marshals who can only control one jack) there is no limit on how many they can control. Its more of a risk/reward choice for you to make. They can't spend the main Warcaster's jack points on their Warjack.
They have the following rules, which I will simplify:
Battlegroup commander
This allows them to control Warjacks in the same way your main Warcaster does. This means that if the Warjack is in their control area it will gain power-up focus and they can allocate focus to it.
Focus manipulation
This works in the same way it does with your main Warcaster - they will replenish focus up to their Focus stat in the Control phase. They have a Control range, which is double their current focus stat and can boost attack and damage rolls, and buy melee attacks. They can also spend focus to shake effects.
Power field
Like a Warcaster they can spend focus during their activation to heal one point of damage per focus spent. They can also spend a focus point to negate 5 damage per attack.
Spellcaster
They can cast spells in the same way your main Warcaster can. So, unlike a solo which must use a star action or attack to cast a spell (for example, a War witch siren or a Greylord forge seer, who have to use their movement before they can cast a spell) a Jr can cast a spell and then advance/charge or advance/charge then cast a spell. (Note they still can't cast a spell and run or cast after failing a charge.)
Spells and battlegroups
Something to be aware of are which spells will or won't work with a Jack under the control of a Jr.
For example Malakov1's (Khador's Jr) spell 'Redline' can only be cast on a Jack he controls as it is limited to a Jack in his battlegroup. If Malakov is included in Irusk1's army, Irusk could cast 'Superiority' on a Jack that Malakov controls as this spell states that it affects "target friendly faction warjack" (though the Jack can't benefit from both Superiority and Redline at the same time as they are both upkeep spells)
Another example: the The Witch Coven can't cast Infernal Machine onto a Jack controlled by Aiakos1 (Cyrx's Jr) as it states "target warjack in the spellcaster's battlegroup"
It's therefore worth checking the spell interactions in your list to see what is possible.
They are affected by the spell 'Backlash' (when a Jack that is part of a battlegroup is damaged its controller suffers one damage point) and the ability feedback. This does the same effect but from a different method, so both happen for a total of two damage to your Jr per damaging attack on the Jack. Because of this you might need to be careful with the Jack your Jr controls when facing these abilities, as they will quickly die by them!
Character Jacks and Bonds
If the Jr you are using is a named character which has a Character Warjack that can be bonded with it (for example Caine0 and Ace) then they will be able to benefit from the bond.
They are also allowed to bring their Bonded warjack with them into any theme list which the Jr is allowed in regardless of whether or not the Warjack is named as a valid choice in that theme.
(So Caine0 would be able to bring Ace into the Heavy Metal theme and gain access to its Rune Shot types from the Bond)
Death of a Jr
So what if the worst happens and your Jr gets killed? Well any Warjacks it was controlling become "inert".
While a Jack is inert it loses any focus it had and can't gain any more. It can't activate, has no melee range and can't engage or be engaged. This means that if an enemy model is in melee with your inert Jack they don't gain the +4 def against ranged and magic attacks for being in combat.
Your inert Jack is auto hit in melee and has def 5 against ranged and magic attacks. They don't gain the armour benefit of shields or bucklers. Inert Jacks don't contest scenario elements.
Reactivating inert Warjacks
You can reactivate your inert Warjack using a model with the Battlegroup commander special rule (your main Warcaster and any Jr's in your list, though currently only Cygnar has access to more than one) or a friendly faction Jack marshal.
A Warcaster or a Jr have to be base to base with the inert Jack and spend one focus, this will reactivate the Jack and put it into their battlegroup.
A Jack marshal has to be base to base with the inert Jack and forfeit their combat action. This will reactivate the Jack and bring it under the control of the Jack marshal (unless they are already in control of a Jack, in which case the reactivated Jack will become Autonomous).
If the Jack that went inert was a colossal then it will not come under the control of a Jack marshal as they can't control them, in this case it will become Autonomous.
When an inert Jack is reactivated it must forfeit its combat action that turn.
In later turns Autonomous Jack can activate as normal but does not benefit from being in a battlegroup or being Marshaled.
If you have an inert Jack that you would like to bring into your main Warcaster's control, but they are too far away to reactivate it, you could have a Jack marshal that already controls a Jack reactivate the inert Jack making it autonomous. Then, have the autonomous Jack advance towards your Warcaster (remembering to forfeit its combat action). Finally have your Warcaster advance into base to base and spend a focus to bring the autonomous Jack into its battlegroup.
And that's the rules that you might need to know about Jr's. I only play Cygnar so I will likely write a tactica for their Jr's at some point. If there is enough interest I could write one about a Jr in another faction but bear in mind that I have no practical experience using the other factions' Jr's so it would be totally theory based!
I hope you found this article helpful.If you did, please share it with your local clubs or drop a comment below - feedback is always welcome, as are suggestions for future articles!
(The OG and most used Jr!)