According to this report, shooter used 10 round magazines.
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http://www.miamiherald.com/news/local/community/broward/article202486304.html
Several state legislators who visited the school with crime-scene investigators said they learned from police that Cruz’s rifle was not top-of-the-line, perhaps explaining the malfunction.
The “weapon and bullets were not high quality and were breaking apart,” one of the legislators, state Sen. Lauren Book, D-Plantation, told the Herald.
Cruz went in with only 10-round magazines because larger clips would not fit in his duffel bag, Book said.
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http://www.miamiherald.com/news/local/community/broward/article202486304.html