Hmm! There were ever really only two bullet weights for the 303. 215 grains or 174 grains. The lighter bullet being the last loading and the one that saw duty through WWI and WWII. Norma did offer a 303 loaded for Kynoch (when Kynoch ammunition was being sold as "loaded in England from Swedish components") with a 150 grain bullet.

The problem that you will hve with your double are that the 303 really only makes the "adverised" 2450fps in a twenty-five inch barrel. So you'll be struggling to get that even in a sorter barrel double rifle. And that is with the 174 grain bullet.

So:

1) Will velocity be a crucial factor in terms of what and where you can hunt? In Britain for example a deer legal rifle MUST have a velocity of a minimum of 2440fps (set in fact to all 303 to be legally used) so regardless of all the arguments about muzzle energy etc., etc. your 215 grain bullet might not be legal in terms of velocity in some jurisdictions.

2) Availablity. Probably the best bullet for the 303 is, in fact, the Norma 150 grain soft point loading. It is long ago in my memory but I think that Hornady offer a "Light Magnum" 303 that is the nearest equivalent. I would avoid the 215 grain bullet...it will be expensive...and stick with either 150 or 174 grain.

3) Speer's offering! We used to use this in Britain when no other 303 bullets were available to the handloader. It was awful! The shape meant that it really did drop like a stone compared with the same weight and same velocity loaded with a "pulled" military spitzer bullet. Avoid! It is useless!

My advice? Unless velocity is a factor choose what is the most likely to be available. That WILL be a bullet of 174 (or 180) grains and choose a spitzer style bullet. If only for use on deer choose a rifle regulated for the modern equivalent of the Norma 150 grain loaded for Kynoch AND LAY IN A LARGE STOCK OF THE SAME BATCH.