Buckhammer74,
I have a 2003 SS Huntsman, as well as several other muzzleloaders. My Huntsman is very accurate with both 209's and the .25 ACP Conversion. The 209's are very messy, the blowback creates a lot carbon build-up after several shots. If you are just using your Huntsman for hunting and not shooting over a dozen shots per range session, you will probably not have to clean the carbon out only 1 or two times before a complete cleaning. This carbon build-up will eventually hinder lock-up and closing issues will occur. The Orange 209 carrriers experience "wash-out" from the first shot, progressively getting worse as more shots are fired to the recommended 20 shots by NEF. I have one carrier that has over 60 (I think 65) shots on it, just to see if I could burn one out to an unusable state. It would still hold a primer and work, but is severly washed-out. I also have three primer carriers that were worthless after one shot with Black Mag'3 and a 460 grain conical. The Winchester primers actually blew apart, locking themselves into the plastic carriers. BM'3 is a progressive powder, and the heat and pressure with those heavy conicals were too much for the carriers. I have shot lighter saboted bullets and up to 100 grains of BM'3 without a problem. I can however shoot the heavy conicals with 777, Pyrodex and APP without any problems to the carriers.
I think there may be a slight improvement in accuracy with the .25 ACP ignition, due to more uniform ignition, but I haven't conducted any definitive tests that would prove the theory. My Huntsman is just accurate, period.
Where the .25 ACP ignition really shines is in the cleanliness department. It is about 95-99% cleaner than the 209's. This not only saves time cleaning carbon out of the receiver, but saves the hidden blowback that you dont see that gets into the internal moving parts of your receiver.
I have tried all the standard 209 primers, 777 primers, Rem Kleanbore and Rem 209-4 (410 primers). I have not tried the NEW TC/Federal Fusion muzzleloading primers, nor will I. I have come to the conclusion that I do not like 209 primers for muzzleloaders.
The .25 ACP is a very good design, but there is room for improvement. I am currently working on a better mouse trap as we speak.