pretty sure it's just the color of the tip. I use the 308 168gr z-max in my 3006 pistol and ar10 308 and they shoot awesome. additionally, you can often find them on sale or closeout because I don't think they went over all that well. I snagged a couple 500rd boxes and couldn't be happier with the performance. They are definitely lightly constructed. I shot a deer with the 168gr Z-Max and I had no exit, no piece of bullet larger than a jacket fragment to recover. BUT... it penetrated to the offside shoulder and the deer fell in it's tracks. approx velocity 2650fps muzzle 200yd shot
Yep tip color ,,,, i asked Hornady myself..... midway had a steal of a deal sale on them a few weeks ago paid less for them than there factory seconds where,,, 12 cent each for 50 gr .224 bought 1500 of them ....
Second hand information: Someone on another forum asked that question to the Hornady rep and was told that the only difference was the tip color, which makes sense.