Might be my imagination but I get the feeling they are different.
I was shooting at a Shiva and doing crap damage to it with the UAC2s, but then my UAC2s ran out of ammo and i was just firing UAC5s, at which point after I compensated, my damage shot up, that seems to me to support that they are slower.
Not sure.
Perhaps start having them in different groupings and save the UAc5s for Shivas?