not sure what a mandrel is, if it's balls it's balls. as far as i am concerned tho, a mandrel bend in round thru the whole length of the bend. i dont know what muffler shops use, but in the electrical trade, our pipe benders by code, have to leave the inside diameter the same as unbent pipe. my bender here in the shop is just par for the course, you can find them at Greenlee's tool site, whatever that is. you can try greenlee.com but it might not work. i also have a bender from G-B which is gardnerbender. i have the mini-egor hydraulic bender. go figure, ive never used it for car exhaust.
try this: http://220.127.116.11/wwwroot/greenlee/bending.pdf