Originally Posted by blueshifty
Are you sure you meant joules...? I haven't seen that unit of measure much outside EE. I'd think Newtons would be more appropriate, but I'm not a physicist.
It's very possible joules are used due to the distance the force travels after impact, which would be a direct correlation to the time the impact is felt.
If paint breaks when a particular force is applied, then the average force the receiver applies to the paint (equal to the average force the paint applies until it breaks) is going to affect the time felt as well. Thus if both .68 and .50 caliber paint travel the same distance into skin the .68 will have a higher joule rating because of it's increased mass. A way to negate that would be to make the .50 caliber paint break after traveling a farther distance after impact, decreasing the brittleness and possibly causing paint to not break.
BTW I'm tired and this might not make any sense when I wake up.