I realize magnum primers are often recommended for use with HS-6 and they may work best for your situation. I use only cast bullets in handgun loads. The limited comparisons using standard CCI versus magnum CCI that I've done with HS-6 in .44 Special loads has shown no difference. That could change with .44 magnum and heavier loads.

However, if you have standard and magnum primers, try both. I've used HS-6 for a good while. Much like Power Pistol, it seems to be a good powder in specialized applications. It's not a particularly versatile propellant, but it probably wasn't designed for that purpose.