Do you have Dave Canterbury's slingbow? I bought one but haven't had a chance to field test it yet. (Too afraid of losing the arrows in the 2 metres of snow we still have!) Dave has the hunter bands that are supposed to be 42# and the HD bands which are supposed to be 50-60# draw which are better suited for deer/antelope hunting.

The other type of spear is the atlatl which outperforms an ordinary spear in accuracy.

Ahh, atlatls! A subject fit for an archaeologist. In days of old (12,000 years ago, more or less) they served to bring down mammoths. I saw a recent study that concluded that an altatl, or spearthrower, increased the force of the spear about 60 times. It is basically an extension of the arm which leverages the force, something like those gadgets that folks use today to throw tennis balls for their dogs. I believe they are outlawed in the major leagues, even for outfielders.

I've had good success with my 165gr spear tip flying out of my 30-06 at 2800 fps.

The idea seems to be to make a spear as an improvised weapon which is what I would plan on doing. Like many things it pays to practice at home. You could do target practice with a javelin, and simple thrust and parry with a stick with a tennis ball on it with a friend. You're unlikely to achieve expert status, but the few minutes or hours learning principles could be valuable. Same applies to other improvised weapons like clubs and thrown weapons.