I have a coupon for 30% off a Shilen barrel and am planning to build a 38-50 Remington on a High Wall action for BPCR shooting.  Shilen shows a .375 barrel with a 12" twist and a black powder barrel with a .379 groove and 20" twist.  Which barrel will work best for my build?

For BPCR, definitely the 12 twist.  20 twist won't stabilize a big enough bullet to go past 250 yards effectively, and won't be heavy enough to knock over rams.  I shoot a couple of 38's with 360 grain bullet and 12 twist.  They are very accurate, and stable and accurate out to past 800 yards.

My wife shot 8 out of 8 on the buffalo at Quigley with her 12 twist 38-55 this year.  After a 2 minute windage sight adjustment after the first shot, she put the next 7 into the white bull.  That's 3 moa at 800 yards.