Wow, That is impressive performance. Years ago I shot a 5x5 (appox. 650lbs) with a 270 and 150 grain bullet at 40 yards. Like your round balls it penetrated fully broadside and stopped under the hide on the other side. The bullet retained about 110-120 grains and now is about .530 in diameter. I think round balls have a significant advantage once contact is made. They retain 90+% of their weight consistently. Only premium (high dollar) bullets compare in weight retention. No doubt 45 cal and up conicals would do the job as well but I am partial to round balls.