Be sure to have the following items before assaulting a warehouse: pistol, shotgun, magnum, Tommy gun, six Molotovs, and about $25,000. Make sure all you guns are fully loaded. When heading toward a warehouse, first try to blow up the roadblocks. Then wait there and headshot oncoming gangsters with your pistol. Now go into the warehouse area and stand behind boxes while shooting at enemies with your pistol or Magnum. Have your shotgun or Tommy gun ready once you actually enter the warehouse since most enemies will also have those weapons. Kill the enemies and find the owner (he will have puppeteer hands). Buy him out, making sure you don’t hit or injure him. Now check your map to find the nearest FBI agent. Bribe him, which may cost you $3,000 but it’s less of a risk. you will now have your own warehouse and avoid a mob war at the same time.
An easy way to take a warehouse is by driving a car into the blockades and running over people. If your car catches on fire, look for a new car and continue running enemies over.