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I'm on the road at the moment, but the short answer is to alloy 0.25-0.50% copper into the mix.  The easiest way is to order from Rotometals.  ( I will look the exact alloy up later) I use a mix of 48lb.s or so of 50/50 pistol range scrap and WW's with a 2 lb. ingot of Rotometal, then add a bit of solder as I go while actually casting.  Smelting in 50lb batches is easy on the old man,,,,.

Experimenting with adding copper actually goes back to at least the opening years of the 1900's.

That one quarter to one half of one percent copper presents multiple advantages.  Increased malleability and toughness under pressure and the ability of to withstand much higher velocities top the list.  When a cast Bullet of this alloy is smashed with a hammer it does not fragment and crack, it merely deforms.  Absolutely what you're looking for in a hunting bullet, yes?

The recent work done was a collaboration among several individuals over on Cast Boolits.  I will try to post links later also.

As Bob and Paul have discovered, the Cu alloy works very well from pedestrian speeds to very high velocities.  Honestly, I've gotten to the point that it's all I use in anything other than revolvers and semi automatic pistols.