What 6.5 brass are you talking about? 6.5x54 MS? 6.5x52 Carcano? 6.5x55 Swedish? 6.5 Jap? 6.5 Grendel? they might look the same upon visual inspection, but I'm willing to bet that unless it's headstamped 6.5x54 or 6.5 MS, it won't go.
You can only use what your barrel is marked for, with some special exceptions. what you're trying to do is the equivalent of chambering a .30-06 cartridge in a .308. "they're both .30's, they should chamber, right?"
There are several 6.5mm cartridges out there, and none of them will interchange with the other.