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There was a bit of a firefight when the 7th Cavalry came upon three Soviet vintage 20 - millimeter anti - aircraft guns , ack - ack guns .
 Soviet, the 7th Cavalry, three Soviet vintage 20 - millimeter anti - aircraft guns, ack - ack guns
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: organization, person, geographical social political, vehicle, location, weapon, facility 
Text: {0} 
Answer:
 Soviet, the 7th Cavalry, three Soviet vintage 20 - millimeter anti - aircraft guns, ack - ack guns