7mm Rolling Block with "modern ammo"

[Jolly Green]

Out of curiosity I was looking at a 7mm Rolling Block. Seeing how Samco is selling 7mm for ~.16, could this mixed modern ammo be used in this old rifle. I know these rifles are notorious for elongated head space and should be checked for that, but is there any other problem? I've read reviews the Samco surplus averages around 3" at 100. Any guesses what it will do out of a RB at 200-300?

Thanks for any responses.