Sweeping !51883 from 21.6 to 22.0-mc20. Make particle gun samples have the correct masses (GeV to MeV change)

Atlas Nightlybuildrequested to merge
cherry-pick-f4ced4e492f-22.0-mc20 into 22.0-mc20

Make particle gun samples have the correct masses (GeV to MeV change)

See merge request !51883 (merged)

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