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We need artillery.
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“...We need artillery....”
In WW2 the most popular weapon in the French Resistance was the small Browning .25 caliber pistol or belly gun. It was easily concealed and its main purpose was to kill a German Soldier to get his better weapons.
In such a situation you have all the artillery you want, you just need to acquire it from those that currently have it. The difficulty will be moving it and acquiring additional ammunition for it. Then you either acquire a supply convoy or a new piece of artillery with ammo.