Ok so I don’t recommend going by weight when shooting powder out of a muzzloader because it’s not how it is used . But I do both and understand it’s not equal. Not something to play with. but I reload and can be anal about powder charges and never liked volume as it leaves room for error depending on how you pack it. For example check your lose powder one day in a tube you can measure 100 grains then tap it ten times and it might be 90. Anyway they say measure the same way everytime and it should be ok. as for Pyrodex I used to shoot 150 grains by volume.
70gr weight BH = 100gr volume. Comparable to 100gr of pellets. I've killed 3 with this load behind 250gr Barnes tez. Two dropped and one ran over a damn cliff and left a blood trail like a murder scene. That should have been enough BH for the bullet to perform.
I have been looking for the Barnes 250 TEZ bullets for some time also with no luck. Have you considered the 290g.? Some folks seem to think they are actually better than the 250g and I have seem them in stock online in a few places.