A rough calculation says that a 1 pound spear thrown at 20 fps would have about the same penetration as our arrows. Well, it would have the same momentum but it has to make a lot bigger hole in the deer for itself. So this only works if the spear has a muzzy point and a diameter equal to that of an arrow. Maybe a solid steel rod pushing a muzzy. Hmmm, just doesn't feel right.

Take a 600 grain arrow at 200 fps. This would equal a 6000 grain spear - 13.71 ounces - at 20 fps. This is just 5.33 foot pounds.

I found a web site that does a comparison. Basically the bow and arrow ranks just below spears in all three categories considered: KE, momentum, and cross sectional density. A bow and arrow would be more accurate and harder to dodge, I'd think.

http://www.thudscave.com:80/npaa/articles/howhard.htm