I don't have a real extensive amount of experience with either the .270 or the 6.5x55mm......maybe a dozen kills with each one.
In my limited experience however, I'd be hard pressed to say one did more "damage" than the other. On the other hand there does seem to be a real difference in penetration. The 6.5 seems to have a real edge in leaving a hole in-hole out type of performance.
In my opinion this is because I've typically used a 140 grain bullet in the 6.5x55 while most often the .270 is loaded with a 130 grain bullet. This has two effects.
First, the 140 grain 6.5mm bullets are much more heavy-for-caliber than the lighter .270 bullets which almost always results in better penetration. Secondly, the 6.5x55 sends its bullets at 2600-2800 fps which is the "perfect" velosity range for cup-and-core bullets while the .270 (at closer range) arrives at something over 3000 fps.....just a bit too much for "normal" bullets.
A switch to a heavier 150 grain bullet in the .270 would solve this problem....not because the bullets are tougher or "better" but because the velosity would drop into the "sweet spot" for normal design bullets. I don't use (or trust) solids (monometal) bullets so I really can't say what, if any effect that might have.
Once I went to 150 grain bullets for the .270 penetration was so much improved. The secret, to me, is choosing a bullet that arives at the target in the 2600-2800 fps "sweet spot".
