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I'm sure you can find a way to sling most rifles along a bike's top tube. Just keep it wrapped up.
I have also seen plenty of people carrying cello cases by bike, if you don't mind an old mafia move.
Considering the realities of modern warfare, an inconspicuous bike touring trailer can transport quite a few drones and grenades.
There's also more conspicuous trailers that can carry up to 500lbs of weapons, simply cover it all up with a tarp.