This really isnt surprising... With very modest muzzle velocities at around 2600, actual impact speeds are low. And using bullets designed as.match bullets, you get really spotty expansion. Its totally a result that should be expected. Compound that bullet expansion issue with a target only 2 inches thick and the bullet expands outside the.critter....
Really explosive expansion is fun, but its usually a product of speed. There is no real way to safely get 3000 fps out of the 6.5/260. Theres only one real choice then: use a very fragile bullet. If you go to 120s, you will gain a little speed however something like Noslers Ballistic Tip or Hornady s Vmax will get it done nicely. As a rule they are far 'softer' and expand well at lower speeds. I would not expect dramatic explosive expansion with a 260 120 Nosler BT at 450 yards. You will see good expansion however.
Side note: grab a good older reload manual. Look up 6.5 JDJ in the handgun section. The preferred deer load was a 120 BT at 2400 fps. They did not explode, but rather expanded well on deer at 50.and 100 yard ranges. Again, low impact speeds. Step the 6.5C or 260 out to long range. Speeds drop, and we are talking very similar impact velocity. Again, expansion not explosion.
If you want dramatic blow em up explosive performance, a 140 6.5mm at 2600 MV cant deliver. You need 2900, 3000 fps or more