The balls concerned are made from a solid rubbery material and designed to be re-used for practice, although I have seen the velcro ones on the net. The guys that make them say "not for shooting at people" and supply target holders with net backs for practicing your shooting skills, not a bad idea if they accurately mimic a paintball in terms of weight etc.
My concern is that as paintball markers are exempt from uk firearms laws because they are designed to fire paintballs, could the introduction of a non-frangible round designed to pass thru a marker change their legal status?
Paintball markers are not firearms because a firearm is defined in law as a "lethal barrelled weapon". Lethality is defined by the projectile's ability to cause a penetrating wound, and anything that can is a firearm. Airsofters are looking at a 328 fps limit due to this legislation, as energy levels at velocities over that level can theoretically cause a penetrating wound. Shooting someone with such a weapon, even with their consent, is defined as assault and punishable accordingly. During the big fuss a few years ago when various police forces etc tried firing a variety of nasty things thru markers, it was agreed that markers are designed to only fire paintballs. It seems a retrograde step to introduce a solid round that is designed to pass thru a "weapon" that, as far as UK law is concerned, can only fire paintballs. Tests have shown that the larger the rubber ball, the less likely it is to cause such a wound, so the .68 balls should be ok, but it might be handy for the industry in the UK to agree a position before any potential problems arise, or shops start stocking these balls, individuals order them over the net etc.
Thanks to everyone for their polite informed comments.