War is mentioned in Revelation many times, but the word guns is questionable...
However:
Matthew 24:6 And ye shall hear of wars and rumours of wars: see that ye be not troubled: for all these things must come to pass, but the end is not yet.
also Luke 21:9
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Note: II Corinthains 10
(For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds) Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;And having in a readiness to revenge all disobedience, when your obedience is fulfilled.

