Correct some powders dont do well reduced. As you seen you dont get full case obturation with a light load , this is why you get gas blow back. Fire forming and neck size is one solution along with some use of faster powders. Theres known published data out there that works so no need to do any testing. H4895 is the only powder Hodgdon claims for thier powders can be safely reduced. For bother cast and jacketed bullets. The use of faster powders help also as it will have that initial peak pressure that expands the case. H4895 works well for me in so many applications. Varget also. So i go with what i can get. Right now varget takes up a lot of shelf space so i use that for most "normal" loads from 223-8mm mauser.