I have blown out a omni viii L3 on a 6.8 while shooting. Not too crazy just controlled leading shots. Then all the sudden I though someone shot back at me with black spots in the tube. Sent it out for warranty and they replaced it at no charge.
Even though my new tube is rated for 6.8 it no longer is mounted to the gun. I just use the IR laser now, plus I can track and hit better with a laser anyways, plus it is faster. Now looking at thermal scope of the weapons sight and the PVS 14 will be for walking, driving, stalking, etc.
As for caliber selection. As Frank has said the bigger calibers have more room for error when pigs are on the run. Plus the heavier 120 gr on 6.8 will knock them down while running or at least slow them down.
Sure 308 pack more of a punch but at a recoil and muzzle rise cost.
As for the 6.5 vs 6.8, they are very close to each other with the 6.8 having a bit more energy under 200 yards and the 6.5 holds it energy better down range. What seals the deal for me on this debate is the 6.8 has more of a selection with ammo, guns, and parts than the 6.5 also the 6.8 bolt is stronger since the case head is smaller on the 6.8.