If this is a "worldwide" battery you have to be aware of both calibres are readily available and what are actually prohibited in certain countries. For that reason ALL military calibres, .223; .308; .30-06 6.5mm Swedish (6.5x55), 7mm Mauser must be discounted. As must ANY rimfire apart from standard .22LR and any relatively "exotic" cartidges such as 7mm-08.

So a .22LR ir is.

That really leaves next only the .270, love it or loathe it. It's available, legal and will hande up to 150 grain bullets in FACTORY ammunition.

A 12 bore, again being aware that some countries prohibit multi-shot guns is next. The simplest probably a Greener's GP Gun. It offers rifle like accuracy, is robust, esily repaired and gives a fast rate of fire.

In the middle calibre it would have to be .375 Holland. Although this is perhaps not as common as it once was.

For the larger calibre, again love or loathe it, it would have to be either .458 Winchester or .416 Rigby. The .416 is more flexible, and probably performs better.

So that's my choice. .22LR; .270; .375; .416; 12 Bore. Not exciting, very "conservative" but I'll at least be able to find ammunition when you are still wasting petrol driving from shop to shop to be told "Not sold here, sorry".