What do you mean by opening up? opening the ends of the Bergers? I shoot a 6.5-300 WBY. IT shoots 140 bergers and 143ELDX, 147M well. Just got a box of 156 EOL in.Extended range is exactly what out to 500 yards is in this country, it's not really long range but your pushing normal gear and should be putting some rounds on your rifles and be ready to go.
I have a pretty good idea what bullets work good on elk in a 6.5, I've been playing with a 7mm lately but last year alone my buddies killed 18 elk with 6.5's most inside 500 yards, most with a 140 Berger all one shot kills, quick and clean. On wanted to try some 127 Barnes and those elk both required tracking and multiple shots with shot placement behind the shoulder, didn't even finish the box of bullets. I don't like 140 Bergers on elk because I read something about the construction I put a LOT of them through elk and then opened them up, I've done the same with a lot of other bullets and I still haven't seen a bullet in 6.5 that will give the consistent terminal performance of the 140 Berger though I'm super stoked to get some 156 Bergers into elk with the bigger 6.5's. I don't really give a hoot about what marketing tells you about bullet construction and what it will do, I'll shoot a few antelope and deer and if that looks good I'll run a few through elk, I open everything I shoot up and take a detailed look at bullet performance which is half the fun in hunting for me.