Years ago - thirty or more - I shot up the few boxes of surplus 6.5x55 ammo I had. Now I load it. Having said that, I've shot 8x56mmR AHE ammo from 1938 in the last couple years without incident.
Consensus seems to be smokeless ammunition lasts a good long time unless subjected to flood or extreme temperature fluctuations.
I had a small pile - maybe on hundred rounds - of old, berdan primed FN head stamped 7x57mm ammo that were cheap as the vendor advertised them as 'from a flooded basement'. The packaging was gone, presumably destroyed. They were discolored and somewhat corroded. I sorted out the corroded ones, then shot - tried to shoot - the only discolored ones. As I recall over half of them fired what seemed to be normally.
So your 6.5 ammo? Unless it's been abused, I would at least try a few. If anything, they won't fire at all (no problem) and at worst, they might lodge a bullet in the barrel. It will not blow up from the powder. (When powder goes bad, it loses power.) Which is easy to tell and a brass rod from a hardware store will remove.
However, remember the primers are corrosive and the barrel needs cleaning soon after shooting. And the cases aren't reloadable for those of us spoiled by boxer primers.
