Thanks Lar45. If you don't mind, try it with a water capacity of 195g. That's basically a little more than .00757"/1" taper. This seems reasonable, and will also mitigate the high bolt thrust with a better friction coefficient (term?).
You could certainly legthen the barrel, but at some point you negate some of the benefit of it being lighter than the BMG. A rail gun might as well weigh 100#, right? I'm surprised at the choice of powders. Figured more in the 7828/H870 range.