I ran into a somewhat similar problem running H380 in my 8mm Mauser and .30-40 Krag. It's just the AMOUNT of unburned stuff in my Ruger #1 that surprises me. Oh, that and the condition of the unburned stuff, it's still in extruded and dark (graphited?) form. I'm sure that for a box of 50 .45-70 IMR3031 and XMR-5744 handloads I could probably recover enough of the stuff to make a whole 'nother round.
In other words, more - lots more - than just a few unburned granules.
Now, the same type of load behind a Speer jacketed flatnose 405gr bullet burns quite clean. So I tend to believe the pressure logic. Hence my trying Alliant 2400 and Reloder 7.
As I finish building my Rolling Block Creedmoor, I planned on using 405-450 gr cast or swaged bullets. I figured with at least a 30" barrel, stuff would burn clean, save for pure FFg loads. Now there's more testing to be done, yee-haa!