Inferno is probably the best paint I have used. Not really used Hellfire much (although it is supposed to be even better than Inferno) but I can't see it being a problem. Now, I ain't no scientist and this info is only based on my own empirical testing...
You need a high quality paint that doesn't have weak seams (cheaper paints usually do), that way with the pressure the Halo creates in the ball stack they don't split before making it to your breech.
But Halo's are also particular about multicoloured and very shiny paint (the eye version ones, not applicable with the sound activated variants) so try and use a single coloured paint (again, like Inferno) for best results.
However non of this negates the fact that there is a great difference in motor powers (manufacturing tolerances seem to be quite wide) and eye quality and therefore some of them are just....poo. If you get a good one that works 90% of the time hold on to it! You might never get another that works that well again
I also thing the push button on off is a major mistake (not such a problem with the Reloader as it blinks green when on) as you can't be sure if it is on or not.