Steve:
My understanding is that early double-based 'powders' like Cordite (which contains nitro-cellulose and nitro-glycerine) were erosive because they burned at much higher temperatures than nitro-cellulose alone.

Also, early powders were not coated to slow-down ignition so probably burned a little hotter in the throat because of that. In other words, modern powders burn cooler because they are progressive burners due to the coating. That includes modern double-based powders too.

That's just my take on things, but if its off the mark I'm ready to be educated!