caliber.co.uk would be a much better domain, as this would relate better to the caliber of a person.
calibers.co.uk is not as good in terms of branding potential. This domain would make most sense in the gun context. Personally I am not in to this sector, but calibers is rather a niche technical term anyway and not really a marketing term.
Where do you see the value in the domain?
EDIT - The UK spelling is calibre, it's the US spelling that is caliber, so that is another issue I'm afraid.