Slugs

[ballisticboy]

A pellet barely contacts the barrel, slugs have 10x more surface to rub against, some designs have thin rings to seal mimicking pellet style contact.

Pellets contact the barrel at the maximum diameter point on the head and at the base of the flare. There is no reason on earth why slugs should not do the same if they are properly designed. 155mm shells do not have the entire side in contact with the barrel and there is no reason a slug should do either. Your assertion that slugs have 10x more surface area is only true for a poorly designed slug based on bullets and not optimised for air guns. If you look at my avatar on other forums you will see a slug with minimal barrel contact area.

Is the big difference between diabolo and slug type pellets not that diabolos are more suited to sub-sonic speeds, and slugs to trans- and super-sonic?
If this is the case it may be why only very powerful airguns (+/- 1125 FPS) really see the benefit from slugs?

Diabolos are more suited to smooth bore which is probably where they originated. There is no reason why aerodynamically subsonic projectiles should be a diabolo shape.