I dont know if your talkign about some particular bullet or what but I've shot solid copper barnes expanders since my dad bought a TC encore 50 cal ML back when they first legalized them in NY... IDK maybe 10-15 years ago. I would not call that 'new' by any means so not sure if your referring to something different.

I must say that the barnes expander in my dads TC with 130gr of pyrodex has taken every deer shot down in a couple seconds, and in somecases 'dumped'. Overal I'd say just as quickly or even more so than my 270. Now I have never shot them myself since I got my own ML but back then I had absolutely no complaints.

I just bought a Savage 220F and just got the scope parts and shells in the mail today and will be using the barnes tipped expanders so we'll see this shotgun season how they work.