I wouldn't expect to get those velocities with that short barrel, I also wouldn't waste the time/powder/bullets trying to get the vari-flame to reliably ignite BH209. I have tried the small rifle primers and the small magnum rifle primers in the .25 ACP with mixed results. The small rifle primers will not give instant ignition, noticable lag time, can hear the hammer fall before ignition. The small rifle magnum primers are a bit better, but not what I would consider 100% reliable. With very tight fitting saboted bullets it works, but with the Hornady FPB's and other conicals it was not working. I haven't actually shot the PowerBelts with it, but I would expect poor results.
If you want reliable ignition every time, get some hot 209 primers such as Federal 209A or CCI Mags.