Pasquot,
I usually use muzzle loaders round balls. It is no trick to jam a .36 or even .45 cal roundball into a 6.5mm barrel. Just set it on the muzzle and pound it in with a plastic mallet. You will cut off the mushroom head just pounding it like a head gasket. I often drive the lead only half a diameter of the ball and pull it back out to get the muzzle measurment - looks like a mushroomed bullet.
Cheers from Grayest California,
Ross