Northern Ireland Police Federation chairman Terry Spence, who represents rank and file officers, praised his "brave" colleagues on their find.
He said the weapons were "undoubtedly intended to murder officers" and that there is "hiding place for terrorists".
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Emergency services were alerted to the crash at 3.50pm on Thursday, Sussex Police said.
WARNING: Some people may find some of the images in this report distressing.
Medical staff who helped during the Westminster terror attack have received a royal thanks for their life-saving efforts.