Well here's one darn good reason;
http://www.midwayusa.com/product/873...oint-box-of-12
I can load those 500's with my own cast lead 440 grain bullets, 50 of them for under 14 bucks! If I were to switch to the 350 Hornady HP, the cost would go up to $8.00+37.99=$45.99 for 50! This is the extreme example though. They are very proud of the 500 ammo, it's a limited production. Not too many people enjoy getting beat up by that cannon.