What cartridge is your Martini chambered for? I know what I read on Wikipedia. Says they were set up for a .323 lead bullet. Seems like anything .308 would rattle down the lands.
I've looked at the NBT 120gr for 7mm, mainly 7-30 Waters and 7mm TCU. It's popular with shooters of those calibers, maybe even the consensus "best" bullet. But the information I've seen is pretty consistent that 1800 fps is the recommended minimum impact velocity for any NBT. So if you give yourself a cushion and keep it above 2000 fps, it doesn't give you much range.
Nosler publishes .366 G1 BC for the .308 125gr BT, which is only a hair better than the .358 Barnes publishes for the 120gr TAC-TX. The 110 gr and 120 gr TAC-TX are both represented to expand as low as 1300-1350 fps (depending on what source you find). If you start it at 2550-2600, which a .30-30 AI with a 21-inch barrel can probably do, the 120 should stay above 1500 fps out to 500 yards. Of course, drop gets crazy and therefore challenging to make ethical hits long before that. The main value of the 500 yards concept is that you just don't have to worry about it opening if you're making shots at 300-ish and in.
