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I know this is old but I've spent time in the army and afterward got involved in work and exercise I would consider beyond anything I experienced there - with a magnet in my hand. I never had problems with the magnet there so I highly doubt I would have in the service. Having shot firearms similarly during that time period, there would have been zero effect from that either.
If for whatever reason a magnet failed and rejected, just cut it out and keep the location gauzed and given neosporin. If anyone asks you got a gash somehow. It won't keep you from doing anything, I had an M31 reject just by design and continued my physical training no problem with bandaging on.