Henry Krank at Pudsey, near Leeds has Swiss Null OB priming powder. I use this in flintlock shotguns. With the correct priming powder and a bit of lock tuning these guns can be made to shoot really well with ignition speeds only a fraction slower than percussion guns and with amazing ignition reliability. Some shotgunners also use Swiss No.1 for priming. My Pedersoli Mortimer shotgun is much faster on ignition with Swiss Null OB than with Swiss No.1. You have to try them and work out what is best for your gun.
Hope this helps.