Yep, those 75gr. Sierras will work well in your new barrel. You should be able to get about the same speed out of that 24" tube as my 10 inch .256, and it will drop groundhogs nicely to 200 yards or so. Try a hefty charge of Lil' Gun with a mag primer and see what you think. In the TC you're only limited by the brass, not the gun so you can get some pretty serious performance out of that 25-20.