Growler Boom makes the Growler, a popular carriage, show up more often. It is one of the cheapest Insider upgrades.
Growler Boom is entry 6 of 9 in the Insider branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £1,500, 25 Metal. Before Growler Boom unlocks you need Medicine Market.
Ale Heist
Ale carts run through the city; hijack or escort them for cash.
Black Market Affiliation
Gun and throwing knife ammunition costs 25% less at every shop.
Carriage Sabotage
Blighter carriages become less resilient.
City-Wide Craze
Halves the price of hallucinogenic darts.