Using the .25 ACP breech plug with CCI small rifle primers, and BH209 caused hang fires for me in my Huntsman.
Using Nick's MU breech plug, I was able to use the same small rifle primers in the Vari-flame adapters to ignite BH209. I was also able to use Win 777 209 primer, Rem Kleenbore ML 209 primer and Rem 209 .410 primer. I did all of this on 80+ degree F so it may behave differently when it is really cold.
I prefer the MU breech plug over the .25 ACP breech plug. Yes, it's that good.
From the load data provided by both companies, T7 FFFg and BH209 seem to have very similar velocities.