I was just wondering why powders like Bullseye, Unique, Blue Dot, Herco, are unsuitable for .223.  Are they too fast or too slow?  What's special about a powder like Reloader 15?

Those are all pistol and shotguns powders you listed, and are fast burning powders.

Those fast pistol powders might be useful if shooting reduced cast bullet loads where just a few grains of powder are used.  I don't think I'd want to shoot such loads out of an AR, however.  A bolt action would be quite fun actually with cast bullets.  


To make a fairly long story short, rifle powders burn slower so that the pressures do not rise too fast.  There is also a lot more energy packed into the case by shear volume and weight of the rifle powder.  This allows for a longer pressure curve which can make use of the longer barrel of the rifle.  In a short pistol barrel, and in a smallish case for caliber, the faster burning powders do better.  There is a faster rise in pressure, but with the small case and large expansion ratio when the bullet moves, the pressures stay within reason.  In addition, bottle neck rifle cases do not allow for that large expansion ratio that a straight wall case gives.  A fast burning powder would quite quickly develop dangerous pressures if more than a small amount were used.

Reloder 15 and similar burning rate and energy dense powders essentially optimize the pressure curve with suitable weight bullets (60 to 80 grain).  I tend to like a bit faster powder like IMR 4895 for the light weight (45 to 53 grain) bullets.

Many thanks.